The differences in types of information-processing skills developed by different instructional backgrounds affect, negatively or positively, the learning of further advanced instructional materials. If prior and subsequent instructional methods are different, a proactive inhibition effect produces low achievement scores on a post test. This poses a serious problem for routing of students to an instructional level on the sole basis of performance on a diagnostic adaptive test and makes it essential to determine what information-processing strategy was used and consider this knowledge simultaneously. Response time often provides supplementary information which differentiates among individuals showing identical quality of performance. A model that reflects this kind of information, obtainable from response time scores, is formulated in a similar manner to latent trait theory. This model is useful in identifying discriminating items that are sensitive to differences in instructional method. It also is helpful in identifying an individual's instructional background to a certain extent. (Author/CTM)