For between-cases groups, each case is in one and only one group. The two groups have different cases. For example, in a medical study patients in one group would receive the treatment and the patients in the other would receive a placebo or alternative treatment; no patient would receive both treatments.
Groups defined by the characteristics of the cases, such as sex, are necessarily between.
For within-cases groups, each case is in both groups. The two groups have the same cases. Examples include before and after scores and repeated measures of the dependent variable. When two cases from each group are different but closely linked, the two groups are treated as within-cases. Examples include data from both members of a couple, twins, and litter mates.
© 2002, Gary McClelland