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'''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 design|study designs]].
'''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. Although [[confounding]] is considered by many authors as a form of bias, it 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]].
==Selection bias==
==Selection bias==
===Case-control studies===
===Case-control studies===
The main source of selection bias in case-control studies is through the selection of the '''control group'''. It is of vital importance that the control group comes from the same population as the case group (that is, if they happened to experience the outcome of interest during the study period, they would have been classified as a case instead of a control)
although the selection of the cases can also result in selection bias.
===Cohort studies===
===Cohort studies===