Rating motivational interviewing fidelity from thin slices.

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...