Abstract
Monitoring fidelity to psychosocial treatments is critical to dissemination, process and outcome research, and internal validity in efficacy trials. However, the costs required to behavior code fidelity to treatments like motivational interviewing (MI) over many therapists and sessions quickly become intractable. Coding less of a session accelerates the process, but it is not clear how much of a session must be evaluated to capture the fidelity of the entire session. The present study used a "thin slice" (Ambady & Rosenthal, 1992) paradigm to explore the degree to which variously sized thin slices of MI fidelity related to fidelity ratings for a full session. We randomly selected contiguous and noncontiguous segments of MI sessions at each whole percent of sessions (i.e., a slice consisting of 1% of session utterances, another at 2%, etc.). We then computed MI fidelity scores from these segments and calculated agreement with fidelity ratings obtained from the full session. We compared thin slice agreement with full sessions against interrater agreement and found that approximately a third of a session (9 min, 26 seconds in our sample) had sufficient agreement to approach interrater levels. These results provide a reference for researchers and clinicians to make efficient and informed use of their behavior coding resources. In addition, our results add to the behavior slicing literature, indicating that small therapist behavior samples adequately describe overall session behavior. (PsycINFO Database Record