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 results in the selection of a study sample which is not representative of the source population, and so affects inclusion of individuals in the study.

Information bias

Information bias results from errors in measurement or classification of individual characteristics of the individuals in the study, which may relate to exposures or outcomes of interest.