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 during analysis, 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]].

Bias can be introduced into a study through the selection of participants ('''selection bias'''), or through errors made in the classification of measurement of exposures/outcomes of interest ('''information bias'''). These biases may also be classified as '''differential''' or '''non-differential''', depending on whether the likelihood of bias is affected by an individual's exposure to factors of interest, or experience of the outcome of interest.

 

As mentioned [[Populations and samples#Sampling strategies|earlier]], the '''validity''' of an estimate is a measure of how well it can be extrapolated to the population in question (or, in the case of ''external validity'', to other populations). Therefore, both selection and information biases will tend to reduce the validity of an estimate, which is of particular concern for any epidemiological study.

==Selection bias==

==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:

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]]:

===Descriptive or cross sectional studies===

===Descriptive or cross sectional studies===