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To date, few studies have been published on the doseresponse relationship, but there is general consensus that between 13 and 18 sessions of therapy are required for 50% of patients to improve. Reviewing the clinical trials literature reveals that in carefully controlled and implemented treatments, between 57.6% and 67.2% of patients improve within an average of 12.7 sessions. Using naturalistic data, however, revealed that the average number of sessions received in a national database of over 6,000 patients was less than five. The rate of improvement in this sample was only about 20%. These results suggest that patients, on average, do not get adequate exposure to psychotherapy, nor do they recover from illness at rates observed in clinical trials research.
Hansen et al. (Tue,) studied this question.