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Measures of strength of association (view source)
Revision as of 07:38, 5 May 2011
, 07:38, 5 May 2011no edit summary
In the case of a [[Study design#Cross sectional studies|cross sectional study]], the '''[[Measures of disease frequency#Prevalence|prevalence]]''' can be estimated amongst exposed individuals as (a<sub>1</sub>/n<sub>1</sub>), and amongst unexposed individuals as (a<sub>0</sub>/n<sub>0</sub>).<br>
In the case of a [[Study design#Cross sectional studies|cross sectional study]], the '''[[Measures of disease frequency#Prevalence|prevalence]]''' can be estimated amongst exposed individuals as (a<sub>1</sub>/n<sub>1</sub>), and amongst unexposed individuals as (a<sub>0</sub>/n<sub>0</sub>).<br>
In the case of a [[Study design#Cohort studies|cohort study]] or a [[Study design#Experimental studies|randomised controlled trial]], and assuming that the disease status relates only to ''new'' cases of disease, the '''[[Measures of disease frequency#Incidence risk|incidence risk]]''' can be estimated amongst exposed individuals as (a<sub>1</sub>/n<sub>1</sub>), and amongst unexposed individuals as (a<sub>0</sub>/n<sub>0</sub>). Alternatively, the [[Measures of disease frequency#Incidence rate|incidence rate]] can be estimated, if the total animal-time for each exposure group is known, as (a<sub>1</sub>/[total number of animal-time units in exposed group]) amongst exposed animals and (a<sub>1</sub>/[total number of animal-time units in unexposed group]) amongst unexposed animals.<br>
In the case of a [[Study design#Cohort studies|cohort study]] or a [[Study design#Experimental studies|experimental study]], the disease status of individuals will relate only to ''new'' cases of disease (i.e. those which were not diseased at the start of the study. In these cases, the '''[[Measures of disease frequency#Incidence risk|incidence risk]]''' can be estimated amongst exposed individuals as (a<sub>1</sub>/n<sub>1</sub>), and amongst unexposed individuals as (a<sub>0</sub>/n<sub>0</sub>). Alternatively, the [[Measures of disease frequency#Incidence rate|incidence rate]] can be estimated, if the total animal-time for each exposure group is known, as (a<sub>1</sub>/[total number of animal-time units in exposed group]) amongst exposed animals and (a<sub>1</sub>/[total number of animal-time units in unexposed group]) amongst unexposed animals.<br>
In the case of a [[Study design#Case control studies|case control study]], no [[Measures of disease frequency|measures of disease frequency]] can be calculated, as selection of individuals was based upon their disease status. However, the '''odds of exposure''' can be estimated amongst diseased individuals as (a<sub>1</sub>/a<sub>0</sub>), and amongst nondiseased individuals as (b<sub>1</sub>/b<sub>0</sub>).<br>
In the case of a [[Study design#Case control studies|case control study]], no [[Measures of disease frequency|measures of disease frequency]] can be calculated, as selection of individuals was based upon their disease status. However an analytic study can still be conducted. This is achieved by looking at the '''odds of exposure''' in the different disease groups. It can be shown that (as long as the ''[[Sampling strategies#Sampling fraction|sampling fraction]]'' is different for cases than controls), the exposure odds ratio comparing diseased to non diseased animals is identical to the odds ratio for disease, comparing exposed to nonexposed animals. The odds ratio amongst diseased individuals is calculated as (a<sub>1</sub>/a<sub>0</sub>), and amongst nondiseased individuals as (b<sub>1</sub>/b<sub>0</sub>).<br>