Compare Pairs examines related samples and makes inferences about the differences between them.
Related samples occur when observations are made on the same set of items or subjects at different times, or when another form of matching has occurred. If the knowing the values in one sample could tell you something about the values in the other sample, then the samples are related.
There are a few different study designs that produce related data. A paired study design takes individual observations from a pair of related subjects. A repeat measures study design takes multiple observations on the same subject. A matched pair study design takes individual observations on multiple subjects that are matched on other covariates. The purpose of matching similar subjects is often to reduce or eliminate the effects of a confounding factor.