Effect size calculations for the clinician: methods and comparability.

OBJECTIVE: The measurement of clinical change via single-group pre-post effect size has become increasingly common in psychotherapy settings that collect practice-based evidence and engage in feedback-informed treatment. Different methods of calculating effect size for the same sample of clients and the same measure can lead to wide-ranging results, reducing interpretability.
METHOD: Effect sizes from therapists-including those drawn from a large web-based database of practicing clinicians-were calculated using nine different methods.
RESULTS: The resulting effect sizes varied significantly depending on the method employed. Differences between measurement methods routinely exceeded 0.40 for individual therapists.
CONCLUSIONS: Three methods for calculating effect sizes are recommended for moderating these differences, including two equations that show promise as valid and practical methods for use by clinicians in professional practice.