A set of observations obtained from a group of subjects and a set of observations obtained from the same group of subjects on a different occasion are paired data sets. The observations form observation-pairs. Unlike the case of unpaired data sets, revelations of the paired data sets are not manifested by comprehensive descriptions of the respective distributions of the data sets but by comprehensive descriptions of the observation-pairs. If the observations are quantitative, revelations of the data sets are clearly manifested by the distribution of a pair index that characterizes the observation-pairs.