Discrete-trial teaching is a strategy frequently used to teach functional skills to individuals with developmental and intellectual disabilities. Research has shown that the within-trial components of the procedure should be administered with ≥90% treatment integrity to facilitate optimal learning. Usually within-trial treatment integrity is measured using whole-session methods such as percentage of trials correctly administered. This study demonstrated one-step Markov transition matrices as a method of assessing within-trial treatment integrity. All components of discrete trials were coded and timestamped from video recordings of therapist-learner dyads in their typical setting (home or school). Several types of within-trial treatment integrity errors were identified using the Markov transition matrices, error sequences that could not be identified using a percentage correct analysis. Better identification of errors has the potential both to enhance treatment integrity and to gain efficiency by targeted retraining of therapists.Discrete trial teaching (DTT) is a strategy frequently used to teach functional skills to individuals with developmental and intellectual disabilities. DTT is a critical component of early intensive behavioral intervention programs. Most skills taught during the initial stages of early intensive behavioral intervention use a DTT format. DTT consists of five distinct within-trial components (Koegel, Russo, & Rincover, 1977). These components are the discriminative stimulus (S D ), a prompt if necessary, a response (correct, incorrect, or no response), a consequence determined by the type of learner response, and an intertrial interval (ITI). Discrete teaching trials have a distinct beginning and end. The presentation of the S D constitutes the beginning of the trial and the start of the ITI the end. In the early stages of teaching, DTT takes place in a structured and controlled environment consisting of one-to-one teaching sessions between therapists and learners. DTT involves breaking down particular skills into smaller parts and teaching each one of these smaller skills individually.utterance to indicate to the learner that the response was incorrect. The prescribed sequence was Incorrect -Error Consequence (p = .28). The two most likely error sequences were full physical guidance prompts (Incorrect -Physical Guidance Figure 1. Percentage treatment integrity for the final 10 blocks of 30 trials during baseline.