Implementation of an adaptive achievement test for teaching signed-numbers operations to junior high students is described. A computer program capable of finding 240 basic errors in signed-number computations was written on the PLATO system and used for analyzing a 64-item conventional test, as well as an adaptive test of addition problems. The item selection procedure consisted of a student answering (either correctly or incorrectly) to the first item administered, and each of two error vectors (for the absolute value and the sign respectively) indicating that there were alternative rules that he/she may have used in getting that answer. Provided the student used a consistent rule of operation, at most four items would determine the rule. If the rule was found to be erroneous, the student was prescribed review exercises. If the student had switched rules from item to item, the errors were considered "random." Correct answers to as many as 27 of the 64 test items needed to be scored as wrong in order to discredit "false corrects," i.e., obtaining correct answers while using wrong rules of operation. (Author/RL)