Changes

'''Systematic error''', or 'bias' is of particular importance in any epidemiological investigation, and should be avoided wherever possible. As with random error, biases will reduce the '''accuracy''' of any results obtained, but they also have the potential to reduce the '''validity''' of results. The forms of bias covered here can only be minimised through careful study design and execution - they cannot be accounted for in the analysis. Although [[confounding]] is considered by many authors as a form of bias, it can be accounted for in the analysis stage, and so is covered separately.<br>

'''Systematic error''', or 'bias' is of particular importance in any epidemiological investigation, and should be avoided wherever possible. As with random error, biases will reduce the '''accuracy''' of any results obtained, but they also have the potential to reduce the '''validity''' of results. The forms of bias covered here can only be minimised through careful study design and execution - they cannot be accounted for in the analysis. Although [[confounding]] is considered by many authors as a form of bias, it can be accounted for during analysis, and so is covered separately.<br>

There are a number of types of bias, which may be classified broadly as either selection bias or information bias, and which will differ in the case of different [[Study design|study designs]].

There are a number of types of bias, which may be classified broadly as either '''selection bias''' or '''information bias''', and which will differ in the case of different [[Study design|study designs]].

==Selection bias==

==Selection bias==

===Non-differental information bias/Measurement error===

===Non-differental information bias/Measurement error===

Non-differential bias occurs when the chance of bias is not affected by the group the individuals belong to. '''Measurement error''' is a common example of non-differential bias - for example, if scales are not correctly calibrated, they will incorrectly record the weight of all animals weighed, regardless of their 'exposure' status. Non-differential bias will tend to reduce the strength of any association present, and will increase the probability of a [[Random variation#Hypothesis testing and study power|type II error]],

Non-differential bias occurs when the chance of bias is not affected by the group the individuals belong to. This type of bias in analytic studies will tend to reduce the strength of any association present, and will increase the probability of a [[Random variation#Hypothesis testing and study power|type II error]]. '''Measurement error''' is a common example of non-differential bias - for example, if scales are not correctly calibrated, they will incorrectly record the weight of all animals weighed, regardless of their 'exposure' status.  

===Differential information bias===

===Differential information bias===

Differential bias occurs when the chance of bias is different for the different groups being compared. For example, Boxer dogs may be more likely than other dog breeds to be diagnosed as having mast cell tumours, even if they do not have them, due to a postulated breed predisposition.

Differential bias occurs when the chance of bias is different for the different groups being compared, and may strengthen or weaken the estimated strength of association in an analytic study. For example, Boxer dogs may be more likely than other dog breeds to be diagnosed as having mast cell tumours, even if they do not have them, due to a postulated breed predisposition.

[[Category:Veterinary Epidemiology - General Concepts|I]]

[[Category:Veterinary Epidemiology - General Concepts|I]]