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===Techniques applied during study design===
===Techniques applied during study design===
====Restriction====
====Restriction====
By restriction of the source population to include only those individuals with the same level of exposure to the confounding variable, confounding due to this variable can be reduced. As this restriction is applied to all individuals included in the study, there should be minimal bias, and internal validity should be good. This is performed to some degree in any study, whenever the source population is defined - for example, a study investigating risk factors for mastitis in cows may be restricted to dairy cattle only, since the cattle production system is likely to be a strong confounder of any associations.
====Matching====
====Matching====
This process involves ensuring that the level of exposure to the confounding variable is the same in the groups being compared, and may be performed on an individual level (matching each animal with at least one other of the same exposure to the confounder - known as ''individual matching'') or on a group level (attempting to make the distribution of exposure to the confounding variable similar in the two groups - known as ''frequency matching''). The effects of matching, and therefore the reasons for using a matching strategy may differ between different study designs, and may or may not be primarily aimed at reduction of confounding. Care also needs to be taken when using matching, as there is a risk of '''overmatching''' (especially in case-control studies), where the true effect of the variable of interest is unable to be measured due to a close association with the matching variable.
===Techniques applied during study analysis===
===Techniques applied during study analysis===