Reported is research related to the use of Computer-Assisted Instruction (CAI) drill-and-practice systems in the elementary schools. The investigators attempted to determine the variables related to problem difficulty by analyzing the solutions of a problem series. Students were first taught the mechanics of utilizing a computer-based teletype to solve arithmetic word problems. Following this, a series of word problems was presented. A central characteristic of these mathematical problems was the requirement of a quantitative answer, but the arithmetical operations were not explicitly indicated. Six factors were analyzed to determine the predictive worth of each for determining problem difficulty. These were the operations variable, the steps variable, the length variable, the sequential variable, the verbal-clue variable, and the conversion variable. The sequential variable was identified as the most important of the six variables. The conversion variables and operations variable were also identified as valuable predictors of the probability of a correct response to each item. (RP)