A study of the responses made to problems selected from Raven's Progressive Matrices showed differences in the kinds of errors typically made by eighteen-year-old students. Nine problems were used and the 68 subjects were told to draw their answers on the answer sheets. There were two trials per puzzle. A classification system, devised for errors, included categories of failure to understand the principle underlying the puzzle and errors due to incomplete application of the correct principle. Thirteen-year-olds' errors indicated a failure to understand the principle underlying the puzzles. Eighteen-year-olds' errors indicated that they understood the principle but applied it incompletely. These results supported the contention that qualitative differences between younger and older adolescents can be inferred from their nonverbal performance alone. In addition, results were consistent with previous findings that older adolescents performed at higher levels than younger adolescents on formal operational problems. (Author/CTM)