WikiVet English - User contributions [en]

Measures of effect and impact

2011-05-19T17:13:08Z

Hannah861: /* Calculating measures of effect and impact */

[[Category:Veterinary Epidemiology - Statistical Methods|D]]
<big>
 
==Calculating measures of effect and impact==
Measures of effect and impact can be calculated using the same contingency table used to calculated measures of strength of association: 

{| class="wikitable"
|-
| Disease status || Exposed || Unexposed || Total
|-
| Diseased || a1 || a0 || m1
|-
| Non-diseased || b1 || b0 || m0
|-
| Total || n1 || n0 || n
|}</big>
<big>
===Risk difference (RD)===
'''Risk difference''' or '''Attributable risk''' is the amount of disease observed in the population which is attributable to the exposure of interest and is calculated by subtracting the risk of disease in the unexposed group from the risk of disease in the exposed group. It assumes that the risk of disease in the non-exposed group is the ‘baseline’ risk of disease, therefore in order to calculate a risk difference groups of animals must be similar with regards to all other factors associated with disease.</big>

[[File:risk difference.jpg]]

<big>

===Attributable fraction (AF)===

The '''attributable fraction''' is the proportion of disease in exposed individuals that is due to the exposure, alternatively, it is the proportion of cases that could have been prevented if exposure had been avoided. It is calculated by dividing the risk difference by the risk of disease in the exposed group, or directly from the risk ratio.</big> 
 

[[File:AF.jpg]]
<big>
===Population attributable risk (PAR)===
The '''population attributable risk''' is analogous to the risk difference in that it gives a difference in the risk between two groups; however PAR is an estimate of the amount of disease in the total '''population''' attributed to the risk factor. It is calculated as the overall risk in the population minus the risk in the unexposed group or by multiplying the risk difference by risk of disease in the population. PAR can only be estimated from studies where the groups selected are representative of the target population.</big> 

[[File:PAR.jpg]]

<big>
===Population attributable fraction (PAF)===
The population attributable fraction is the proportion of all cases in the population that could have been prevented by removal of exposure. It is related to the AF above and is calculated by dividing the PAR by the prevalence of disease in the study population.</big> 

[[File:PAF.jpg]]

Measures of effect and impact

2011-05-19T17:12:56Z

Hannah861:

[[Category:Veterinary Epidemiology - Statistical Methods|D]]
<big>
==Calculating measures of effect and impact==
Measures of effect and impact can be calculated using the same contingency table used to calculated measures of strength of association: 

{| class="wikitable"
|-
| Disease status || Exposed || Unexposed || Total
|-
| Diseased || a1 || a0 || m1
|-
| Non-diseased || b1 || b0 || m0
|-
| Total || n1 || n0 || n
|}</big>
<big>
===Risk difference (RD)===
'''Risk difference''' or '''Attributable risk''' is the amount of disease observed in the population which is attributable to the exposure of interest and is calculated by subtracting the risk of disease in the unexposed group from the risk of disease in the exposed group. It assumes that the risk of disease in the non-exposed group is the ‘baseline’ risk of disease, therefore in order to calculate a risk difference groups of animals must be similar with regards to all other factors associated with disease.</big>

[[File:risk difference.jpg]]

<big>

===Attributable fraction (AF)===

The '''attributable fraction''' is the proportion of disease in exposed individuals that is due to the exposure, alternatively, it is the proportion of cases that could have been prevented if exposure had been avoided. It is calculated by dividing the risk difference by the risk of disease in the exposed group, or directly from the risk ratio.</big> 
 

[[File:AF.jpg]]
<big>
===Population attributable risk (PAR)===
The '''population attributable risk''' is analogous to the risk difference in that it gives a difference in the risk between two groups; however PAR is an estimate of the amount of disease in the total '''population''' attributed to the risk factor. It is calculated as the overall risk in the population minus the risk in the unexposed group or by multiplying the risk difference by risk of disease in the population. PAR can only be estimated from studies where the groups selected are representative of the target population.</big> 

[[File:PAR.jpg]]

<big>
===Population attributable fraction (PAF)===
The population attributable fraction is the proportion of all cases in the population that could have been prevented by removal of exposure. It is related to the AF above and is calculated by dividing the PAR by the prevalence of disease in the study population.</big> 

[[File:PAF.jpg]]

Measures of effect and impact

2011-05-19T17:11:52Z

Hannah861:

[[Category:Veterinary Epidemiology - Statistical Methods|D]]
<big>
Measures of effect and impact can be calculated using the same contingency table used to calculated measures of strength of association: 

{| class="wikitable"
|-
| Disease status || Exposed || Unexposed || Total
|-
| Diseased || a1 || a0 || m1
|-
| Non-diseased || b1 || b0 || m0
|-
| Total || n1 || n0 || n
|}</big>
<big>
===Risk difference (RD)===
'''Risk difference''' or '''Attributable risk''' is the amount of disease observed in the population which is attributable to the exposure of interest and is calculated by subtracting the risk of disease in the unexposed group from the risk of disease in the exposed group. It assumes that the risk of disease in the non-exposed group is the ‘baseline’ risk of disease, therefore in order to calculate a risk difference groups of animals must be similar with regards to all other factors associated with disease.</big>

[[File:risk difference.jpg]]

<big>

===Attributable fraction (AF)===

The '''attributable fraction''' is the proportion of disease in exposed individuals that is due to the exposure, alternatively, it is the proportion of cases that could have been prevented if exposure had been avoided. It is calculated by dividing the risk difference by the risk of disease in the exposed group, or directly from the risk ratio.</big> 
 

[[File:AF.jpg]]
<big>
===Population attributable risk (PAR)===
The '''population attributable risk''' is analogous to the risk difference in that it gives a difference in the risk between two groups; however PAR is an estimate of the amount of disease in the total '''population''' attributed to the risk factor. It is calculated as the overall risk in the population minus the risk in the unexposed group or by multiplying the risk difference by risk of disease in the population. PAR can only be estimated from studies where the groups selected are representative of the target population.</big> 

[[File:PAR.jpg]]

<big>
===Population attributable fraction (PAF)===
The population attributable fraction is the proportion of all cases in the population that could have been prevented by removal of exposure. It is related to the AF above and is calculated by dividing the PAR by the prevalence of disease in the study population.</big> 

[[File:PAF.jpg]]

Measures of effect and impact

2011-05-19T17:03:59Z

Hannah861: /* Population attributable risk (PAR) */

[[Category:Veterinary Epidemiology - Statistical Methods|D]]

<big>
{| class="wikitable"
|-
| Disease status || Exposed || Unexposed || Total
|-
| Diseased || a1 || a0 || m1
|-
| Non-diseased || b1 || b0 || m0
|-
| Total || n1 || n0 || n
|}</big>
<big>
===Risk difference (RD)===
'''Risk difference''' or '''Attributable risk''' is the amount of disease observed in the population which is attributable to the exposure of interest and is calculated by subtracting the risk of disease in the unexposed group from the risk of disease in the exposed group. It assumes that the risk of disease in the non-exposed group is the ‘baseline’ risk of disease, therefore in order to calculate a risk difference groups of animals must be similar with regards to all other factors associated with disease.</big>

[[File:risk difference.jpg]]

<big>

===Attributable fraction (AF)===

The '''attributable fraction''' is the proportion of disease in exposed individuals that is due to the exposure, alternatively, it is the proportion of cases that could have been prevented if exposure had been avoided. It is calculated by dividing the risk difference by the risk of disease in the exposed group, or directly from the risk ratio.</big> 
 

[[File:AF.jpg]]
<big>
===Population attributable risk (PAR)===
The '''population attributable risk''' is analogous to the risk difference in that it gives a difference in the risk between two groups; however PAR is an estimate of the amount of disease in the total '''population''' attributed to the risk factor. It is calculated as the overall risk in the population minus the risk in the unexposed group or by multiplying the risk difference by risk of disease in the population. PAR can only be estimated from studies where the groups selected are representative of the target population.</big> 

[[File:PAR.jpg]]

<big>
===Population attributable fraction (PAF)===
The population attributable fraction is the proportion of all cases in the population that could have been prevented by removal of exposure. It is related to the AF above and is calculated by dividing the PAR by the prevalence of disease in the study population.</big> 

[[File:PAF.jpg]]

Measures of effect and impact

2011-05-19T17:03:08Z

Hannah861: /* Population attributable risk (PAR) */

Measures of effect and impact

2011-05-19T17:02:46Z

Hannah861: /* Population attributable risk (PAR) */

Measures of effect and impact

2011-05-19T17:02:22Z

Hannah861: /* Population attributable risk (PAR) */

[[Category:Veterinary Epidemiology - Statistical Methods|D]]

<big>
{| class="wikitable"
|-
| Disease status || Exposed || Unexposed || Total
|-
| Diseased || a1 || a0 || m1
|-
| Non-diseased || b1 || b0 || m0
|-
| Total || n1 || n0 || n
|}</big>
<big>
===Risk difference (RD)===
'''Risk difference''' or '''Attributable risk''' is the amount of disease observed in the population which is attributable to the exposure of interest and is calculated by subtracting the risk of disease in the unexposed group from the risk of disease in the exposed group. It assumes that the risk of disease in the non-exposed group is the ‘baseline’ risk of disease, therefore in order to calculate a risk difference groups of animals must be similar with regards to all other factors associated with disease.</big>

[[File:risk difference.jpg]]

<big>

===Attributable fraction (AF)===

The '''attributable fraction''' is the proportion of disease in exposed individuals that is due to the exposure, alternatively, it is the proportion of cases that could have been prevented if exposure had been avoided. It is calculated by dividing the risk difference by the risk of disease in the exposed group, or directly from the risk ratio.</big> 
 

[[File:AF.jpg]]
<big>
===Population attributable risk (PAR)===
 
The '''population attributable risk''' is analogous to the risk difference in that it gives a difference in the risk between two groups; however PAR is an estimate of the amount of disease in the total '''population''' attributed to the risk factor. It is calculated as the overall risk in the population minus the risk in the unexposed group or by multiplying the risk difference by risk of disease in the population. PAR can only be estimated from studies where the groups selected are representative of the target population. </big>
[[File:PAR.jpg]]

[[File:PAF.jpg]]

Measures of effect and impact

2011-05-19T17:02:10Z

Hannah861: /* Population attributable risk (PAR) */

[[Category:Veterinary Epidemiology - Statistical Methods|D]]

<big>
{| class="wikitable"
|-
| Disease status || Exposed || Unexposed || Total
|-
| Diseased || a1 || a0 || m1
|-
| Non-diseased || b1 || b0 || m0
|-
| Total || n1 || n0 || n
|}</big>
<big>
===Risk difference (RD)===
'''Risk difference''' or '''Attributable risk''' is the amount of disease observed in the population which is attributable to the exposure of interest and is calculated by subtracting the risk of disease in the unexposed group from the risk of disease in the exposed group. It assumes that the risk of disease in the non-exposed group is the ‘baseline’ risk of disease, therefore in order to calculate a risk difference groups of animals must be similar with regards to all other factors associated with disease.</big>

[[File:risk difference.jpg]]

<big>

===Attributable fraction (AF)===

The '''attributable fraction''' is the proportion of disease in exposed individuals that is due to the exposure, alternatively, it is the proportion of cases that could have been prevented if exposure had been avoided. It is calculated by dividing the risk difference by the risk of disease in the exposed group, or directly from the risk ratio.</big> 
 

[[File:AF.jpg]]
<big>
===Population attributable risk (PAR)===
 

The '''population attributable risk''' is analogous to the risk difference in that it gives a difference in the risk between two groups; however PAR is an estimate of the amount of disease in the total '''population''' attributed to the risk factor. It is calculated as the overall risk in the population minus the risk in the unexposed group or by multiplying the risk difference by risk of disease in the population. PAR can only be estimated from studies where the groups selected are representative of the target population. </big>
[[File:PAR.jpg]]

[[File:PAF.jpg]]

Measures of effect and impact

2011-05-19T17:01:55Z

Hannah861: /* Attributable fraction (AF) */

[[Category:Veterinary Epidemiology - Statistical Methods|D]]

<big>
{| class="wikitable"
|-
| Disease status || Exposed || Unexposed || Total
|-
| Diseased || a1 || a0 || m1
|-
| Non-diseased || b1 || b0 || m0
|-
| Total || n1 || n0 || n
|}</big>
<big>
===Risk difference (RD)===
'''Risk difference''' or '''Attributable risk''' is the amount of disease observed in the population which is attributable to the exposure of interest and is calculated by subtracting the risk of disease in the unexposed group from the risk of disease in the exposed group. It assumes that the risk of disease in the non-exposed group is the ‘baseline’ risk of disease, therefore in order to calculate a risk difference groups of animals must be similar with regards to all other factors associated with disease.</big>

[[File:risk difference.jpg]]

<big>

===Attributable fraction (AF)===

The '''attributable fraction''' is the proportion of disease in exposed individuals that is due to the exposure, alternatively, it is the proportion of cases that could have been prevented if exposure had been avoided. It is calculated by dividing the risk difference by the risk of disease in the exposed group, or directly from the risk ratio.</big> 
 

[[File:AF.jpg]]
<big>
===Population attributable risk (PAR)===

The '''population attributable risk''' is analogous to the risk difference in that it gives a difference in the risk between two groups; however PAR is an estimate of the amount of disease in the total '''population''' attributed to the risk factor. It is calculated as the overall risk in the population minus the risk in the unexposed group or by multiplying the risk difference by risk of disease in the population. PAR can only be estimated from studies where the groups selected are representative of the target population. </big>
[[File:PAR.jpg]]

[[File:PAF.jpg]]

Measures of effect and impact

2011-05-19T17:01:26Z

Hannah861: /* Attributable fraction (AF) */

[[Category:Veterinary Epidemiology - Statistical Methods|D]]

<big>
{| class="wikitable"
|-
| Disease status || Exposed || Unexposed || Total
|-
| Diseased || a1 || a0 || m1
|-
| Non-diseased || b1 || b0 || m0
|-
| Total || n1 || n0 || n
|}</big>
<big>
===Risk difference (RD)===
'''Risk difference''' or '''Attributable risk''' is the amount of disease observed in the population which is attributable to the exposure of interest and is calculated by subtracting the risk of disease in the unexposed group from the risk of disease in the exposed group. It assumes that the risk of disease in the non-exposed group is the ‘baseline’ risk of disease, therefore in order to calculate a risk difference groups of animals must be similar with regards to all other factors associated with disease.</big>

[[File:risk difference.jpg]]

<big>

===Attributable fraction (AF)===

The '''attributable fraction''' is the proportion of disease in exposed individuals that is due to the exposure, alternatively, it is the proportion of cases that could have been prevented if exposure had been avoided. It is calculated by dividing the risk difference by the risk of disease in the exposed group, or directly from the risk ratio.</big> 
 

[[File:AF.jpg]]

<big>===Population attributable risk (PAR)===

The '''population attributable risk''' is analogous to the risk difference in that it gives a difference in the risk between two groups; however PAR is an estimate of the amount of disease in the total '''population''' attributed to the risk factor. It is calculated as the overall risk in the population minus the risk in the unexposed group or by multiplying the risk difference by risk of disease in the population. PAR can only be estimated from studies where the groups selected are representative of the target population. </big>
[[File:PAR.jpg]]

[[File:PAF.jpg]]

Measures of effect and impact

2011-05-19T17:01:01Z

Hannah861: /* Population attributable risk (PAR) */

[[Category:Veterinary Epidemiology - Statistical Methods|D]]

<big>
{| class="wikitable"
|-
| Disease status || Exposed || Unexposed || Total
|-
| Diseased || a1 || a0 || m1
|-
| Non-diseased || b1 || b0 || m0
|-
| Total || n1 || n0 || n
|}</big>
<big>
===Risk difference (RD)===
'''Risk difference''' or '''Attributable risk''' is the amount of disease observed in the population which is attributable to the exposure of interest and is calculated by subtracting the risk of disease in the unexposed group from the risk of disease in the exposed group. It assumes that the risk of disease in the non-exposed group is the ‘baseline’ risk of disease, therefore in order to calculate a risk difference groups of animals must be similar with regards to all other factors associated with disease.</big>

[[File:risk difference.jpg]]

<big>

===Attributable fraction (AF)===

The '''attributable fraction''' is the proportion of disease in exposed individuals that is due to the exposure, alternatively, it is the proportion of cases that could have been prevented if exposure had been avoided. It is calculated by dividing the risk difference by the risk of disease in the exposed group, or directly from the risk ratio.</big> 
 

[[File:AF.jpg]]

===Population attributable risk (PAR)===

The '''population attributable risk''' is analogous to the risk difference in that it gives a difference in the risk between two groups; however PAR is an estimate of the amount of disease in the total '''population''' attributed to the risk factor. It is calculated as the overall risk in the population minus the risk in the unexposed group or by multiplying the risk difference by risk of disease in the population. PAR can only be estimated from studies where the groups selected are representative of the target population. 

[[File:PAR.jpg]]

[[File:PAF.jpg]]

Measures of effect and impact

2011-05-19T17:00:40Z

Hannah861: /* Attributable fraction (AF) */

[[Category:Veterinary Epidemiology - Statistical Methods|D]]

<big>
{| class="wikitable"
|-
| Disease status || Exposed || Unexposed || Total
|-
| Diseased || a1 || a0 || m1
|-
| Non-diseased || b1 || b0 || m0
|-
| Total || n1 || n0 || n
|}</big>
<big>
===Risk difference (RD)===
'''Risk difference''' or '''Attributable risk''' is the amount of disease observed in the population which is attributable to the exposure of interest and is calculated by subtracting the risk of disease in the unexposed group from the risk of disease in the exposed group. It assumes that the risk of disease in the non-exposed group is the ‘baseline’ risk of disease, therefore in order to calculate a risk difference groups of animals must be similar with regards to all other factors associated with disease.</big>

[[File:risk difference.jpg]]

<big>

===Attributable fraction (AF)===

The '''attributable fraction''' is the proportion of disease in exposed individuals that is due to the exposure, alternatively, it is the proportion of cases that could have been prevented if exposure had been avoided. It is calculated by dividing the risk difference by the risk of disease in the exposed group, or directly from the risk ratio.</big> 
 

[[File:AF.jpg]]

===Population attributable risk (PAR)===

The '''population attributable risk''' is analogous to the risk difference in that it gives a difference in the risk between two groups; however PAR is an estimate of the amount of disease in the total '''population''' attributed to the risk factor. It is calculated as the overall risk in the population minus the risk in the unexposed group or by multiplying the risk difference by risk of disease in the population. PAR can only be estimated from studies where the groups selected are representative of the target population.
 

[[File:PAR.jpg]]

[[File:PAF.jpg]]

Measures of effect and impact

2011-05-19T16:50:22Z

Hannah861:

Measures of effect and impact

2011-05-19T16:50:08Z

Hannah861:

Measures of effect and impact

2011-05-19T16:49:41Z

Hannah861: /* Attributable fraction (AF) */

[[Category:Veterinary Epidemiology - Statistical Methods|D]]

{| class="wikitable"
|-
| Disease status || Exposed || Unexposed || Total
|-
| Diseased || a1 || a0 || m1
|-
| Non-diseased || b1 || b0 || m0
|-
| Total || n1 || n0 || n
|}

<big>
===Risk difference (RD)===
'''Risk difference''' or '''Attributable risk''' is the amount of disease observed in the population which is attributable to the exposure of interest and is calculated by subtracting the risk of disease in the unexposed group from the risk of disease in the exposed group. It assumes that the risk of disease in the non-exposed group is the ‘baseline’ risk of disease, therefore in order to calculate a risk difference groups of animals must be similar with regards to all other factors associated with disease.</big>

[[File:risk difference.jpg]]

<big>

===Attributable fraction (AF)===

The '''attributable fraction''' is the proportion of disease in exposed individuals that is due to the exposure, alternatively, it is the proportion of cases that could have been prevented if exposure had been avoided. It is calculated by dividing the risk difference by the risk of disease in the exposed group, or directly from the risk ratio.</big> 
 

[[File:AF.jpg]]

[[File:PAR.jpg]]

[[File:PAF.jpg]]

Measures of effect and impact

2011-05-19T16:49:19Z

Hannah861: /* Risk difference (RD) */

[[Category:Veterinary Epidemiology - Statistical Methods|D]]

{| class="wikitable"
|-
| Disease status || Exposed || Unexposed || Total
|-
| Diseased || a1 || a0 || m1
|-
| Non-diseased || b1 || b0 || m0
|-
| Total || n1 || n0 || n
|}

<big>
===Risk difference (RD)===
'''Risk difference''' or '''Attributable risk''' is the amount of disease observed in the population which is attributable to the exposure of interest and is calculated by subtracting the risk of disease in the unexposed group from the risk of disease in the exposed group. It assumes that the risk of disease in the non-exposed group is the ‘baseline’ risk of disease, therefore in order to calculate a risk difference groups of animals must be similar with regards to all other factors associated with disease.</big>

[[File:risk difference.jpg]]

<big>

===Attributable fraction (AF)===

The '''attributable fraction''' is the proportion of disease in exposed individuals that is due to the exposure, alternatively, it is the proportion of cases that could have been prevented if exposure had been avoided. It is calculated by dividing the risk difference by the risk of disease in the exposed group, or directly from the risk ratio.</big> 

[[File:AF.jpg]]

[[File:PAR.jpg]]

[[File:PAF.jpg]]

Measures of effect and impact

2011-05-19T16:49:03Z

Hannah861:

[[Category:Veterinary Epidemiology - Statistical Methods|D]]

{| class="wikitable"
|-
| Disease status || Exposed || Unexposed || Total
|-
| Diseased || a1 || a0 || m1
|-
| Non-diseased || b1 || b0 || m0
|-
| Total || n1 || n0 || n
|}

<big>
===Risk difference (RD)===
 
'''Risk difference''' or '''Attributable risk''' is the amount of disease observed in the population which is attributable to the exposure of interest and is calculated by subtracting the risk of disease in the unexposed group from the risk of disease in the exposed group. It assumes that the risk of disease in the non-exposed group is the ‘baseline’ risk of disease, therefore in order to calculate a risk difference groups of animals must be similar with regards to all other factors associated with disease.</big>

[[File:risk difference.jpg]]

<big>

===Attributable fraction (AF)===

The '''attributable fraction''' is the proportion of disease in exposed individuals that is due to the exposure, alternatively, it is the proportion of cases that could have been prevented if exposure had been avoided. It is calculated by dividing the risk difference by the risk of disease in the exposed group, or directly from the risk ratio.</big> 

[[File:AF.jpg]]

[[File:PAR.jpg]]

[[File:PAF.jpg]]

Measures of effect and impact

2011-05-19T16:48:43Z

Hannah861: /* Risk difference (RD) */

[[Category:Veterinary Epidemiology - Statistical Methods|D]]

{| class="wikitable"
|-
| Disease status || Exposed || Unexposed || Total
|-
| Diseased || a1 || a0 || m1
|-
| Non-diseased || b1 || b0 || m0
|-
| Total || n1 || n0 || n
|}

<big>
===Risk difference (RD)=== 

'''Risk difference''' or '''Attributable risk''' is the amount of disease observed in the population which is attributable to the exposure of interest and is calculated by subtracting the risk of disease in the unexposed group from the risk of disease in the exposed group. It assumes that the risk of disease in the non-exposed group is the ‘baseline’ risk of disease, therefore in order to calculate a risk difference groups of animals must be similar with regards to all other factors associated with disease.</big>

[[File:risk difference.jpg]]

<big>

===Attributable fraction (AF)===

The '''attributable fraction''' is the proportion of disease in exposed individuals that is due to the exposure, alternatively, it is the proportion of cases that could have been prevented if exposure had been avoided. It is calculated by dividing the risk difference by the risk of disease in the exposed group, or directly from the risk ratio.</big> 

[[File:AF.jpg]]

[[File:PAR.jpg]]

[[File:PAF.jpg]]

Measures of effect and impact

2011-05-19T16:48:25Z

Hannah861: /* Attributable fraction (AF) */

[[Category:Veterinary Epidemiology - Statistical Methods|D]]

{| class="wikitable"
|-
| Disease status || Exposed || Unexposed || Total
|-
| Diseased || a1 || a0 || m1
|-
| Non-diseased || b1 || b0 || m0
|-
| Total || n1 || n0 || n
|}

<big>
===Risk difference (RD)===

'''Risk difference''' or '''Attributable risk''' is the amount of disease observed in the population which is attributable to the exposure of interest and is calculated by subtracting the risk of disease in the unexposed group from the risk of disease in the exposed group. It assumes that the risk of disease in the non-exposed group is the ‘baseline’ risk of disease, therefore in order to calculate a risk difference groups of animals must be similar with regards to all other factors associated with disease.</big>

[[File:risk difference.jpg]]

<big>
===Attributable fraction (AF)===

The '''attributable fraction''' is the proportion of disease in exposed individuals that is due to the exposure, alternatively, it is the proportion of cases that could have been prevented if exposure had been avoided. It is calculated by dividing the risk difference by the risk of disease in the exposed group, or directly from the risk ratio.</big> 

[[File:AF.jpg]]

[[File:PAR.jpg]]

[[File:PAF.jpg]]

Measures of effect and impact

2011-05-19T16:47:53Z

Hannah861: /* Attributable fraction (AF) */

[[Category:Veterinary Epidemiology - Statistical Methods|D]]

{| class="wikitable"
|-
| Disease status || Exposed || Unexposed || Total
|-
| Diseased || a1 || a0 || m1
|-
| Non-diseased || b1 || b0 || m0
|-
| Total || n1 || n0 || n
|}

<big>
===Risk difference (RD)===

'''Risk difference''' or '''Attributable risk''' is the amount of disease observed in the population which is attributable to the exposure of interest and is calculated by subtracting the risk of disease in the unexposed group from the risk of disease in the exposed group. It assumes that the risk of disease in the non-exposed group is the ‘baseline’ risk of disease, therefore in order to calculate a risk difference groups of animals must be similar with regards to all other factors associated with disease.</big>

[[File:risk difference.jpg]]

<big>
===Attributable fraction (AF)===

The '''attributable fraction''' is the proportion of disease in exposed individuals that is due to the exposure, alternatively, it is the proportion of cases that could have been prevented if exposure had been avoided. It is calculated by dividing the risk difference by the risk of disease in the exposed group, or directly from the risk ratio.</big>

[[File:AF.jpg]]

[[File:PAR.jpg]]

[[File:PAF.jpg]]

Measures of effect and impact

2011-05-19T16:47:39Z

Hannah861:

[[Category:Veterinary Epidemiology - Statistical Methods|D]]

{| class="wikitable"
|-
| Disease status || Exposed || Unexposed || Total
|-
| Diseased || a1 || a0 || m1
|-
| Non-diseased || b1 || b0 || m0
|-
| Total || n1 || n0 || n
|}

<big>
===Risk difference (RD)===

'''Risk difference''' or '''Attributable risk''' is the amount of disease observed in the population which is attributable to the exposure of interest and is calculated by subtracting the risk of disease in the unexposed group from the risk of disease in the exposed group. It assumes that the risk of disease in the non-exposed group is the ‘baseline’ risk of disease, therefore in order to calculate a risk difference groups of animals must be similar with regards to all other factors associated with disease.</big>

[[File:risk difference.jpg]]

<big>
===Attributable fraction (AF)===

The '''attributable fraction''' is the proportion of disease in exposed individuals that is due to the exposure, alternatively, it is the proportion of cases that could have been prevented if exposure had been avoided. It is calculated by dividing the risk difference by the risk of disease in the exposed group, or directly from the risk ratio.</big>
[[File:AF.jpg]]

[[File:PAR.jpg]]

[[File:PAF.jpg]]

Measures of effect and impact

2011-05-19T16:37:47Z

Hannah861:

[[Category:Veterinary Epidemiology - Statistical Methods|D]]

{| class="wikitable"
|-
| Disease status || Exposed || Unexposed || Total
|-
| Diseased || a1 || a0 || m1
|-
| Non-diseased || b1 || b0 || m0
|-
| Total || n1 || n0 || n
|}

<big>
===Risk difference (RD) ===

'''Risk difference''' or '''Attributable risk''' is the amount of disease observed in the population which is attributable to the exposure of interest and is calculated by subtracting the risk of disease in the unexposed group from the risk of disease in the exposed group. It assumes that the risk of disease in the non-exposed group is the ‘baseline’ risk of disease, therefore in order to calculate a risk difference groups of animals must be similar with regards to all other factors associated with disease.</big>

[[File:risk difference.jpg]]

[[File:AF.jpg]]

[[File:PAR.jpg]]

[[File:PAF.jpg]]

Measures of effect and impact

2011-05-19T16:37:18Z

Hannah861:

[[Category:Veterinary Epidemiology - Statistical Methods|D]]

{| class="wikitable"
|-
| Disease status || Exposed || Unexposed || Total
|-
| Diseased || a1 || a0 || m1
|-
| Non-diseased || b1 || b0 || m0
|-
| Total || n1 || n0 || n
|}

<big>
===Risk difference=== (RD) 

'''Risk difference''' or '''Attributable risk''' is the amount of disease observed in the population which is attributable to the exposure of interest and is calculated by subtracting the risk of disease in the unexposed group from the risk of disease in the exposed group. It assumes that the risk of disease in the non-exposed group is the ‘baseline’ risk of disease, therefore in order to calculate a risk difference groups of animals must be similar with regards to all other factors associated with disease.</big>

[[File:risk difference.jpg]]

[[File:AF.jpg]]

[[File:PAR.jpg]]

[[File:PAF.jpg]]

File:PAF.jpg

2011-05-19T16:35:35Z

Hannah861: {{Information |Description =Population attributable fraction formula |Source =Microsoft Word |Author =Hannah Holt |Date = |Permission = |other_versions = }}

== Summary ==
{{Information
|Description =Population attributable fraction formula
|Source =Microsoft Word
|Author =Hannah Holt
|Date =
|Permission =
|other_versions =
}}

== Licensing ==
{{PD-self}}

Measures of effect and impact

2011-05-19T16:21:47Z

Hannah861:

File:AF.jpg

2011-05-19T16:18:31Z

Hannah861: {{Information |Description =Attributable fraction formula |Source =Microsoft Word |Author =Hannah Holt |Date = |Permission = |other_versions = }}

== Summary ==
{{Information
|Description =Attributable fraction formula
|Source =Microsoft Word
|Author =Hannah Holt
|Date =
|Permission =
|other_versions =
}}

== Licensing ==
{{PD-self}}

Measures of effect and impact

2011-05-19T16:17:45Z

Hannah861:

Measures of effect and impact

2011-05-19T16:14:56Z

Hannah861:

Measures of effect and impact

2011-05-19T16:11:02Z

Hannah861:

File:PAR.jpg

2011-05-19T16:10:09Z

Hannah861: {{Information |Description =Population attributable risk formula |Source =Microsoft Word |Author =Hannah Holt |Date = |Permission = |other_versions = }}

== Summary ==
{{Information
|Description =Population attributable risk formula
|Source =Microsoft Word
|Author =Hannah Holt
|Date =
|Permission =
|other_versions =
}}

== Licensing ==
{{PD-self}}

Measures of effect and impact

2011-05-19T16:09:26Z

Hannah861:

File:Risk difference.jpg

2011-05-19T16:06:32Z

Hannah861: {{Information |Description =Risk difference formula |Source =Microsoft Word |Author =Hannah Holt |Date = |Permission = |other_versions = }}

== Summary ==
{{Information
|Description =Risk difference formula
|Source =Microsoft Word
|Author =Hannah Holt
|Date =
|Permission =
|other_versions =
}}

== Licensing ==
{{PD-self}}

Measures of effect and impact

2011-05-19T16:04:32Z

Hannah861:

Measures of effect and impact

2011-05-19T15:53:00Z

Hannah861:

Evaluation of diagnostic tests

2011-05-03T15:25:01Z

Hannah861: /* Introduction */

==Introduction==
 

A diagnostic test is an objective method of deciding whether an animal has a disease, or not. Decisions made following diagnostic testing are usually dichotomous e.g. treat or do not treat the animal, therefore diagnostic tests are usually interpreted as dichotomous outcomes (diseased or non-diseased). In this case, if a diagnostic test is measuring a continuous outcome e.g. antibody titre then a cut-off for classifying animal’s as positive or negative must be selected. The figure below shows that whereever the cut-off is selected there is usually some overlap between results i.e. some diseased animals will have the same value as non-diseased animals and resulting in some false-positive and false-negative results.

[[File:Diagnostic test results.jpg]]

==Interpretation of diagnostic tests==
Several statistics can aid clinicians in the interpretation of diagnostic tests these include '''sensitivity''' and '''specificity''', '''predictive values''', '''likelihood ratios''' and '''pre-''' and '''post-test probabilities'''. The following 2 x 2 table is often used by epidemiologists to calculate these statistics:

[[File:table.jpg]]

===Sensitivity and specificity===

'''Sensitivity (''Se'')''' is the probability that a positive animal is correctly identified as positive and '''specificity (''Sp'')''' is the probability a negative animal is correctly identified as negative.

[[File:specificity.jpg]]

===Predictive values===

There are two types of predictive value; '''positive predictive value (PV+)''' is defined as the probability that an animal which tests positive actually has the disease and '''negative predictive value (PV-)''' is the probability and animal that tests negative does not have the disease. PV+ and PV- are calculated as follows:

[[File:predictive value.jpg]]

===Likelihood ratio===

Likelihood ratios (LR) give clinicians an idea of how likely a test result could have been produced by a diseased compared with a non-diseased animals and can be calculated for both positive and negative test results. The LR for a positive test result (LR+) is interpreted as how likely it is to find a positive test result in diseased compared with non-diseased individuals and LR- is interpreted as how likely it is to find a negative test result in diseased compared with non-diseased animals. Likelihood ratios are a characteristic of the test and do not depend on prevalence, they are calculated as follows:

[[File:likelihood ratio.jpg]]

===Post-test probability===
Before performing a diagnostic test a veterinarian usually has an idea of the likelihood that an animal has a disease, usually based on the levels of the disease in the population. The probability that an animal has a disease before a diagnostic test is performed is termed the '''pre-test probability''' and is usually the prevalence of the disease in the population, but can be modified depending on other factors e.g. whether the animal is showing symptoms of disease, whether certain risk factors for disease are present. Once a diagnostic test has been performed this probability can be modified to incorporate the results of the diagnostic tests to give an overall probability that an animal has the disease i.e. '''post-test probability.''' This is carried out as follows:

[[File:post-test probability.jpg]]

File:Specificity.jpg

2011-05-03T15:23:58Z

Hannah861: {{Information |Description =formulas |Source =Microsoft word |Author =Hannah Holt |Date = |Permission = |other_versions = }}

== Summary ==
{{Information
|Description =formulas
|Source =Microsoft word
|Author =Hannah Holt
|Date =
|Permission =
|other_versions =
}}

== Licensing ==
{{PD-self}}

Evaluation of diagnostic tests

2011-05-03T15:22:05Z

Hannah861: /* Sensitivity and specificity */

==Introduction==
 

Decisions made following diagnostic testing are usually dichotomous e.g. treat or do not treat the animal, therefore diagnostic tests are usually interpreted as dichotomous outcomes (diseased or non-diseased). In this case, if a diagnostic test is measuring a continuous outcome e.g. antibody titre then a cut-off for classifying animal’s as positive or negative must be selected. The figure below shows that whereever the cut-off is selected there is usually some overlap between results i.e. some diseased animals will have the same value as non-diseased animals and resulting in some false-positive and false-negative results.

[[File:Diagnostic test results.jpg]]

==Interpretation of diagnostic tests==
Several statistics can aid clinicians in the interpretation of diagnostic tests these include '''sensitivity''' and '''specificity''', '''predictive values''', '''likelihood ratios''' and '''pre-''' and '''post-test probabilities'''. The following 2 x 2 table is often used by epidemiologists to calculate these statistics:

[[File:table.jpg]]

===Sensitivity and specificity===

'''Sensitivity (''Se'')''' is the probability that a positive animal is correctly identified as positive and '''specificity (''Sp'')''' is the probability a negative animal is correctly identified as negative.

[[File:specificity.jpg]]

===Predictive values===

There are two types of predictive value; '''positive predictive value (PV+)''' is defined as the probability that an animal which tests positive actually has the disease and '''negative predictive value (PV-)''' is the probability and animal that tests negative does not have the disease. PV+ and PV- are calculated as follows:

[[File:predictive value.jpg]]

===Likelihood ratio===

Likelihood ratios (LR) give clinicians an idea of how likely a test result could have been produced by a diseased compared with a non-diseased animals and can be calculated for both positive and negative test results. The LR for a positive test result (LR+) is interpreted as how likely it is to find a positive test result in diseased compared with non-diseased individuals and LR- is interpreted as how likely it is to find a negative test result in diseased compared with non-diseased animals. Likelihood ratios are a characteristic of the test and do not depend on prevalence, they are calculated as follows:

[[File:likelihood ratio.jpg]]

===Post-test probability===
Before performing a diagnostic test a veterinarian usually has an idea of the likelihood that an animal has a disease, usually based on the levels of the disease in the population. The probability that an animal has a disease before a diagnostic test is performed is termed the '''pre-test probability''' and is usually the prevalence of the disease in the population, but can be modified depending on other factors e.g. whether the animal is showing symptoms of disease, whether certain risk factors for disease are present. Once a diagnostic test has been performed this probability can be modified to incorporate the results of the diagnostic tests to give an overall probability that an animal has the disease i.e. '''post-test probability.''' This is carried out as follows:

[[File:post-test probability.jpg]]

File:Sensitivity.jpg

2011-05-03T15:21:18Z

Hannah861: uploaded a new version of "File:Sensitivity.jpg": {{Information |Description =formula |Source =microsoft word |Author =Hannah Holt |Date = |Permission = |other_versions = }}

== Summary ==
{{Information
|Description =Formula to calculate sensitivity and specificity
|Source =Microsoft word equation editor
|Author =Hannah Holt
|Date =
|Permission =
|other_versions =
}}

== Licensing ==
{{PD-self}}

File:Sensitivity.jpg

2011-05-03T15:20:03Z

Hannah861: uploaded a new version of "File:Sensitivity.jpg": {{Information |Description =Formula for sensitivity and specificity |Source =Microsoft word equation editor |Author =Hannah Holt |Date = |Permission = |other_

File:Sensitivity.jpg

2011-05-03T15:18:44Z

Hannah861: uploaded a new version of "File:Sensitivity.jpg":

File:Sensitivity.jpg

2011-05-03T15:17:24Z

Hannah861: {{Information |Description =Formula to calculate sensitivity and specificity |Source =Microsoft word equation editor |Author =Hannah Holt |Date = |Permission = |other_versions = }}

Evaluation of diagnostic tests

2011-05-03T15:16:33Z

Hannah861: /* Sensitivity and specificity */

==Introduction==
 

Decisions made following diagnostic testing are usually dichotomous e.g. treat or do not treat the animal, therefore diagnostic tests are usually interpreted as dichotomous outcomes (diseased or non-diseased). In this case, if a diagnostic test is measuring a continuous outcome e.g. antibody titre then a cut-off for classifying animal’s as positive or negative must be selected. The figure below shows that whereever the cut-off is selected there is usually some overlap between results i.e. some diseased animals will have the same value as non-diseased animals and resulting in some false-positive and false-negative results.

[[File:Diagnostic test results.jpg]]

==Interpretation of diagnostic tests==
Several statistics can aid clinicians in the interpretation of diagnostic tests these include '''sensitivity''' and '''specificity''', '''predictive values''', '''likelihood ratios''' and '''pre-''' and '''post-test probabilities'''. The following 2 x 2 table is often used by epidemiologists to calculate these statistics:

[[File:table.jpg]]

===Sensitivity and specificity===

'''Sensitivity (''Se'')''' is the probability that a positive animal is correctly identified as positive and '''specificity (''Sp'')''' is the probability a negative animal is correctly identified as negative.

[[File:sensitivity.jpg]]

===Predictive values===

There are two types of predictive value; '''positive predictive value (PV+)''' is defined as the probability that an animal which tests positive actually has the disease and '''negative predictive value (PV-)''' is the probability and animal that tests negative does not have the disease. PV+ and PV- are calculated as follows:

[[File:predictive value.jpg]]

===Likelihood ratio===

Likelihood ratios (LR) give clinicians an idea of how likely a test result could have been produced by a diseased compared with a non-diseased animals and can be calculated for both positive and negative test results. The LR for a positive test result (LR+) is interpreted as how likely it is to find a positive test result in diseased compared with non-diseased individuals and LR- is interpreted as how likely it is to find a negative test result in diseased compared with non-diseased animals. Likelihood ratios are a characteristic of the test and do not depend on prevalence, they are calculated as follows:

[[File:likelihood ratio.jpg]]

===Post-test probability===
Before performing a diagnostic test a veterinarian usually has an idea of the likelihood that an animal has a disease, usually based on the levels of the disease in the population. The probability that an animal has a disease before a diagnostic test is performed is termed the '''pre-test probability''' and is usually the prevalence of the disease in the population, but can be modified depending on other factors e.g. whether the animal is showing symptoms of disease, whether certain risk factors for disease are present. Once a diagnostic test has been performed this probability can be modified to incorporate the results of the diagnostic tests to give an overall probability that an animal has the disease i.e. '''post-test probability.''' This is carried out as follows:

[[File:post-test probability.jpg]]

File:Predictive value.jpg

2011-05-03T14:56:39Z

Hannah861: {{Information |Description =Formulas for calculating positive and negative predictive values |Source =Microsoft Word equation editor |Author =Hannah Holt |Date = |Permission = |other_versions = }}

== Summary ==
{{Information
|Description =Formulas for calculating positive and negative predictive values
|Source =Microsoft Word equation editor
|Author =Hannah Holt
|Date =
|Permission =
|other_versions =
}}

== Licensing ==
{{PD-self}}

Evaluation of diagnostic tests

2011-05-03T14:55:39Z

Hannah861:

==Introduction==
 

Decisions made following diagnostic testing are usually dichotomous e.g. treat or do not treat the animal, therefore diagnostic tests are usually interpreted as dichotomous outcomes (diseased or non-diseased). In this case, if a diagnostic test is measuring a continuous outcome e.g. antibody titre then a cut-off for classifying animal’s as positive or negative must be selected. The figure below shows that whereever the cut-off is selected there is usually some overlap between results i.e. some diseased animals will have the same value as non-diseased animals and resulting in some false-positive and false-negative results.

[[File:Diagnostic test results.jpg]]

==Interpretation of diagnostic tests==
Several statistics can aid clinicians in the interpretation of diagnostic tests these include '''sensitivity''' and '''specificity''', '''predictive values''', '''likelihood ratios''' and '''pre-''' and '''post-test probabilities'''. The following 2 x 2 table is often used by epidemiologists to calculate these statistics:

[[File:table.jpg]]

===Sensitivity and specificity===

'''Sensitivity (''Se'')''' is the probability that a positive animal is correctly identified as positive and '''specificity (''Sp'')''' is the probability a negative animal is correctly identified as negative.

===Predictive values===

There are two types of predictive value; '''positive predictive value (PV+)''' is defined as the probability that an animal which tests positive actually has the disease and '''negative predictive value (PV-)''' is the probability and animal that tests negative does not have the disease. PV+ and PV- are calculated as follows:

[[File:predictive value.jpg]]

===Likelihood ratio===

Likelihood ratios (LR) give clinicians an idea of how likely a test result could have been produced by a diseased compared with a non-diseased animals and can be calculated for both positive and negative test results. The LR for a positive test result (LR+) is interpreted as how likely it is to find a positive test result in diseased compared with non-diseased individuals and LR- is interpreted as how likely it is to find a negative test result in diseased compared with non-diseased animals. Likelihood ratios are a characteristic of the test and do not depend on prevalence, they are calculated as follows:

[[File:likelihood ratio.jpg]]

===Post-test probability===
Before performing a diagnostic test a veterinarian usually has an idea of the likelihood that an animal has a disease, usually based on the levels of the disease in the population. The probability that an animal has a disease before a diagnostic test is performed is termed the '''pre-test probability''' and is usually the prevalence of the disease in the population, but can be modified depending on other factors e.g. whether the animal is showing symptoms of disease, whether certain risk factors for disease are present. Once a diagnostic test has been performed this probability can be modified to incorporate the results of the diagnostic tests to give an overall probability that an animal has the disease i.e. '''post-test probability.''' This is carried out as follows:

[[File:post-test probability.jpg]]

Category:Veterinary Epidemiology - General Concepts

2011-05-03T14:51:46Z

Hannah861: removed Category:Diagnostic testing using HotCat

{{frontpage
|pagetitle = Veterinary Epidemiology - General Concepts
|pagebody =Veterinary epidemiology is principally concerned with the study of disease within populations (although it may also be used for investigation of issues such as animal welfare and productivity). Put simply, it involves the investigation of patterns of disease within a population, in relation to '''which animals''' are affected, the '''spatial distribution''' (i.e. location) of affected animals, and the '''temporal distribution''' of affected animals (i.e. patterns of disease through time). 
|contenttitle = Contents
|contentbody =
<big>'''<categorytree mode=pages>Veterinary Epidemiology - General Concepts</categorytree>

|logo = epivetnet.gif
}}

[[Category:WikiEpi|1]] [[Category:Epidemiology]]

Category:Veterinary Epidemiology - General Concepts

2011-05-03T14:50:47Z

Hannah861: added Category:Diagnostic testing using HotCat

Evaluation of diagnostic tests

2011-05-03T14:13:37Z

Hannah861: /* Predictive values */

==Introduction==
 

Decisions made following diagnostic testing are usually dichotomous e.g. treat or do not treat the animal, therefore diagnostic tests are usually interpreted as dichotomous outcomes (diseased or non-diseased). In this case, if a diagnostic test is measuring a continuous outcome e.g. antibody titre then a cut-off for classifying animal’s as positive or negative must be selected. The figure below shows that whereever the cut-off is selected there is usually some overlap between results i.e. some diseased animals will have the same value as non-diseased animals and resulting in some false-positive and false-negative results.

[[File:Diagnostic test results.jpg]]

==Interpretation of diagnostic tests==
Several statistics can aid clincians in the interpretation of diagnostic tests these include '''sensitivity''' and '''specificity''', '''predictive values''', '''likelihood ratios''' and '''pre-''' and '''post-test probabilities'''. The following 2 x 2 table is often used by epidemiologists to calculate these statistics:

[[File:table.jpg]]

===Sensitivity and specificity===

'''Sensitivity (''Se'')''' is the probability that a positive animal is correctly identified as positive and '''specificity (''Sp'')''' is the probability a negative animal is correctly identified as negative.

===Predictive values===

There are two types of predictive value; '''positive predictive value (PV+)''' is defined as the probability that an animal which tests positive actually has the disease and '''negative predictive value (PV-)''' is the probability and animal that tests negative does not have the disease. PV+ and PV- are calculated as follows:

===Likelihood ratio===

Likelihood ratios (LR) give clinicians an idea of how likely a test result could have been produced by a diseased compared with a non-diseased animals and can be calculated for both positive and negative test results. The LR for a positive test result (LR+) is interpreted as how likely it is to find a positive test result in diseased compared with non-diseased individuals and LR- is interpreted as how likely it is to find a negative test result in diseased compared with non-diseased animals. Likelihood ratios are a characteristic of the test and do not depend on prevalence, they are calculated as follows:

[[File:likelihood ratio.jpg]]

===Post-test probability===
Before performing a diagnostic test a veterinarian usually has an idea of the likelihood that an animal has a disease, usually based on the levels of the disease in the population. The probability that an animal has a disease before a diagnostic test is performed is termed the '''pre-test probability''' and is usually the prevalence of the disease in the population, but can be modified depending on other factors e.g. whether the animal is showing symptoms of disease, whether certain risk factors for disease are present. Once a diagnostic test has been performed this probability can be modified to incorporate the results of the diagnostic tests to give an overall probability that an animal has the disease i.e. '''post-test probability.''' This is carried out as follows:

[[File:post-test probability.jpg]]

File:Table.jpg

2011-05-03T14:09:50Z

Hannah861: {{Information |Description =2 x 2 table used to calculate test charachteristics |Source =Microsoft Works |Author =Hannah Holt |Date = |Permission = |other_versions = }}

== Summary ==
{{Information
|Description =2 x 2 table used to calculate test charachteristics
|Source =Microsoft Works
|Author =Hannah Holt
|Date =
|Permission =
|other_versions =
}}

== Licensing ==
{{PD-self}}

Evaluation of diagnostic tests

2011-05-03T14:08:42Z

Hannah861: /* Interpretation of diagnostic tests */

==Introduction==
 

Decisions made following diagnostic testing are usually dichotomous e.g. treat or do not treat the animal, therefore diagnostic tests are usually interpreted as dichotomous outcomes (diseased or non-diseased). In this case, if a diagnostic test is measuring a continuous outcome e.g. antibody titre then a cut-off for classifying animal’s as positive or negative must be selected. The figure below shows that whereever the cut-off is selected there is usually some overlap between results i.e. some diseased animals will have the same value as non-diseased animals and resulting in some false-positive and false-negative results.

[[File:Diagnostic test results.jpg]]

==Interpretation of diagnostic tests==
Several statistics can aid clincians in the interpretation of diagnostic tests these include '''sensitivity''' and '''specificity''', '''predictive values''', '''likelihood ratios''' and '''pre-''' and '''post-test probabilities'''. The following 2 x 2 table is often used by epidemiologists to calculate these statistics:

[[File:table.jpg]]

===Sensitivity and specificity===

'''Sensitivity (''Se'')''' is the probability that a positive animal is correctly identified as positive and '''specificity (''Sp'')''' is the probability a negative animal is correctly identified as negative.

===Predictive values===

===Likelihood ratio===

Likelihood ratios (LR) give clinicians an idea of how likely a test result could have been produced by a diseased compared with a non-diseased animals and can be calculated for both positive and negative test results. The LR for a positive test result (LR+) is interpreted as how likely it is to find a positive test result in diseased compared with non-diseased individuals and LR- is interpreted as how likely it is to find a negative test result in diseased compared with non-diseased animals. Likelihood ratios are a characteristic of the test and do not depend on prevalence, they are calculated as follows:

[[File:likelihood ratio.jpg]]

===Post-test probability===
Before performing a diagnostic test a veterinarian usually has an idea of the likelihood that an animal has a disease, usually based on the levels of the disease in the population. The probability that an animal has a disease before a diagnostic test is performed is termed the '''pre-test probability''' and is usually the prevalence of the disease in the population, but can be modified depending on other factors e.g. whether the animal is showing symptoms of disease, whether certain risk factors for disease are present. Once a diagnostic test has been performed this probability can be modified to incorporate the results of the diagnostic tests to give an overall probability that an animal has the disease i.e. '''post-test probability.''' This is carried out as follows:

[[File:post-test probability.jpg]]

Evaluation of diagnostic tests

2011-05-03T14:05:31Z

Hannah861: /* Interpretation of diagnostic tests */

==Introduction==
 

Decisions made following diagnostic testing are usually dichotomous e.g. treat or do not treat the animal, therefore diagnostic tests are usually interpreted as dichotomous outcomes (diseased or non-diseased). In this case, if a diagnostic test is measuring a continuous outcome e.g. antibody titre then a cut-off for classifying animal’s as positive or negative must be selected. The figure below shows that whereever the cut-off is selected there is usually some overlap between results i.e. some diseased animals will have the same value as non-diseased animals and resulting in some false-positive and false-negative results.

[[File:Diagnostic test results.jpg]]

==Interpretation of diagnostic tests==
Several statistics can aid clincians in the interpretation of diagnostic tests these include '''sensitivity''' and '''specificity''', '''predictive values''', '''likelihood ratios''' and '''pre-''' and '''post-test probabilities'''.

===Sensitivity and specificity===

'''Sensitivity (''Se'')''' is the probability that a positive animal is correctly identified as positive and '''specificity (''Sp'')''' is the probability a negative animal is correctly identified as negative.

===Predictive values===

===Likelihood ratio===

Likelihood ratios (LR) give clinicians an idea of how likely a test result could have been produced by a diseased compared with a non-diseased animals and can be calculated for both positive and negative test results. The LR for a positive test result (LR+) is interpreted as how likely it is to find a positive test result in diseased compared with non-diseased individuals and LR- is interpreted as how likely it is to find a negative test result in diseased compared with non-diseased animals. Likelihood ratios are a characteristic of the test and do not depend on prevalence, they are calculated as follows:

[[File:likelihood ratio.jpg]]

===Post-test probability===
Before performing a diagnostic test a veterinarian usually has an idea of the likelihood that an animal has a disease, usually based on the levels of the disease in the population. The probability that an animal has a disease before a diagnostic test is performed is termed the '''pre-test probability''' and is usually the prevalence of the disease in the population, but can be modified depending on other factors e.g. whether the animal is showing symptoms of disease, whether certain risk factors for disease are present. Once a diagnostic test has been performed this probability can be modified to incorporate the results of the diagnostic tests to give an overall probability that an animal has the disease i.e. '''post-test probability.''' This is carried out as follows:

[[File:post-test probability.jpg]]