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. Biases will reduce the '''validity''' of any results obtained, whether it be by overestimating or underestimating the frequency of disease in a population or the association between an exposure and disease. 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>

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.

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.

==Validity==

==Validity==

As mentioned [[Sampling strategies#Populations and samples|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.

As mentioned [[Sampling strategies#Populations and samples|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). The same concept applies to analytic studies, although the validity here relates to the ability to estimate the association between an exposure and an outcome of interest. Both selection and information biases can reduce the validity of an estimate.<br>

 

Due to different investigation aims, it should be noted that introduction of biases into descriptive analytic studies differs from that in analytic studies. In descriptive studies, any process which results in the selection of a population which is not representative of the source population can be said to bias the results of the study. However, in the case of analytic study, the representation of the source population is less important , as long as the measure of association between the exposure and outcome is not affected. Therefore, both exposure and outcome should be considered - if selection is associated with both of these (for example, if selection of 'diseased' individuals was affected by their exposure category), then bias will result. If selection is 'only' associated with 'either' exposure status or outcome, then bias will not result.

==Selection bias==

==Selection bias==

   

   

===Cross sectional studies===

===Cross sectional studies===

Although non-response is also the main route of selection bias entering these studies, it should be noted here that analytic studies differ from descriptive studies in how bias can occur, due to the aim of the study. In these cases, both exposure and outcome should be considered - if selection is associated with both of these (meaning that selection of 'diseased' individuals is affected by their exposure category), then bias will result. If selection is 'only' associated with 'either' exposure status or outcome, then bias will not result.

Non-response is also the main route of selection bias entering these studies,

===Case-control studies===

===Case-control studies===