The stability of a measured psychological function may be assessed by comparing the scores obtained by the same subjects on two or more separate occasions. Three observable effects of instability on the score distribution are (i) change in the mean, (ii) change in the variance, and (iii) change in the ranking, i.e., lack of perfect retest correlations even after proper attenuation corrections. This report is concerned with methods for summarizing, displaying and accounting for large quantities of such data, for a variety of age- levels and time-spans.