The computer programs described in this manual were designed for the analysis of data resulting from psychological experiments which hypothesize an aptitude- or trait-treatment interaction (ATI). The programs were written in Fortran IV for use on most CDC 6400/6600 and IBM 360 computer systems. Research designs to which the programs are applicable represent paradigms which test for aptitude-by-treatment interactions under four conditions: when one continuous aptitude has been measured that is linearly related to the criterion (program ATILIN1); when two continuous attitudes have been measured that are linearly related to the criterion (program ATILIN2); when one continuous aptitude has been measured that is curvilinearly related to the criterion (program ATICURV); and when one aptitude has been measured that is linearly related to the criterion and group sizes are sufficiently large to permit a blocked analysis of variance on individuals with extreme scores in the aptitude distribution (program XGROUPS). Reference to technical and design issues is made in methodological notes which accompany each program. A short annotated bibliography appears at the end of the manual. (CH)