Sudden gains---large, enduring reductions in symptom intensity from one session to the next--were identified by T. Z. Tang and R. J. DeRubeis (1999) on the basis of data from 2 manualized clinical trials of cognitive therapy for depression. The authors found similar sudden gains among clients with a variety of disorders treated with a variety of approaches in routine clinic settings. Clients (N=135 who met inclusion criteria) completed short forms of the Clinical Outcomes in Routine Evaluation (CORE-SF) preceding 7 to 74 individual sessions. Those who experienced sudden gains within their first 16 sessions (n=23) had significantly lower CORE-SF scores in their final 3 sessions than did the other clients.
Stiles, W., Leach, C., Barkham, M., Lucock, M., Iveson, S., Shapiro, D. A., ... Hardy, G. E. (2003). Early sudden gains in psychotherapy under routine clinic conditions: Practice-based evidence. Journal of Consulting and Clinical Psychology, 71(1), 14-21. https://doi.org/10.1037/0022-006X.71.1.14