Bias

Systematic error, or 'bias' is of particular importance in any epidemiological investigation, and should be avoided wherever possible. Biases will reduce the validity or accuracy of any results obtained. 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 designs.

Selection bias

Selection bias affects inclusion of individuals in the study and results in the study sample not being representative of the source population. This may occur in the initial selection process, or may be a result of nonresponse or losses to follow up during the study period. The mechanisms by which selection bias can enter a study will depend on the study design:

Information bias

Information bias results from errors in measurement or classification of exposures or outcomes of interest amongst the individuals included in the study. In the case of analytic studies, this may be classified as differential or non-differential:

• Differential bias occurs when the chance of bias is different for the different groups being compared. For example, in a study investigating whether Boxer dogs have a higher incidence of mast cell tumours than other breeds, Boxers may be more likely to be diagnosed by vets as having these, due to a postulated breed disposition.
• Non-differential bias occurs when the chance of bias is not affected by the group the individuals belong to.

Non-differential bias will tend to reduce the strength of any association present, and will increase the probability of a type II error, whereas the effect of differential bias cannot be predicted.