In an experimental treatment study, the comparison group is exposed to conditions comparable to the experimental group, except for the condition that is investigated regarding its effect.
Experimental treatment studies are designed to estimate the effect of a particular treatment on one or more variables. Typically, the variables of interest are observed before and after treatment to detect changes that occurred in between. The two observations of the variables are called pretest and posttest to indicate their temporal position before and after the treatment. However, any differences between pre- and posttest need not be caused by the treatment. Therefore, experimental treatment studies use at least two groups: the experimental group receives the treatment while the comparison group receives a placebo treatment (or a passive control group that does not receive any treatment at all). The effect of the treatment can be estimated by.
