Comparison group

From EvaluationWiki

A group of comparable subjects used to provide a 'running baseline' against which any changes in the experimental group can be compared in order to identify the effects of the experimental treatment. The most desirable comparison group, in most cases, would be a group containing one of each of a set of identical twins who have been brought up together, since they are matched on genetic as well as social variables, but this is rarely possible. Next best is a group matched on some broad characteristics such as age and gender, but otherwise chosen at random from the same population as the experimental group. Again, while it is ideal to give the comparison group no treatment at all, this tends to cause them to see a mixture of other treatments for their common condition, which makes legitimate inference to the effects of the treatment under study very difficult. So a common compromise is to provide the comparison group with what appears to be a similar treatment but is in fact missing the key element under study: this results in the so-called placebo group for comparison.