Problems in the measurement of gain in adult literacy programs were investigated through repeated testing of a group of students in adult basic education and General Educational Development classes and through computer simulations. Ninety-two students were tested at 3 different times over 7 months with a battery of norm-referenced reading and mathematics tests as well as with tests of reading rate and decoding developed especially for this study. Gain scores were found to vary across tests, with significant declines as well as gains. No significant differences in gains were found for amount of instructional time or for attendance rate, and a large amount of group heterogeneity was revealed through an analysis of growth patterns. Computer simulations for grade-equivalent stability showed that with populations smaller than 200, inconsistencies in grade-level intervals could account for a major portion of the yearly gain typically reported for adult literacy instruction. In contrast, simulations of regression to the mean caused by guessing on multiple-choice tests showed that this effect was relatively small. These results strongly supported the need to construct a multiple indicator system for evaluating adult literacy programs, a system that attends to the multiple goals of such programs and is free of elementary and secondary level conventions such as grade-equivalent scores. (Appended are 12 tables and 3 figures.) Contains 29 references.