Sudden gains are relatively large, quick, stable drops in symptom scores during treatment of
depression that may (or may not) signal important therapeutic events. We review what is known and
unknown currently about the prevalence, causes, and outcomes of sudden gains. We argue that valid identification of
sudden gains (vs. random fluctuations in symptoms and gradual gains) is prerequisite to their understanding. In Monte
Carlo simulations, three popular criterion sets showed inadequate power to detect sudden gains and many false positives
due to (a) testing multiple intervals for sudden gains, (b) finite retest reliability of symptom measures, and (c) failure to
account for gradual gains. Sudden gains in published clinical datasets appear similar in form and frequency to false
positives in the simulations. We discuss the need to develop psychometrically sound methods to detect sudden gains and
to differentiate sudden from random and gradual gains.
Keywords: Assessment, depression, psychotherapy, sudden gains, symptoms.
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