LONG-TERM BENEFITS OF PSYCHOTHERAPY AND THE DIFFERENTIAL SIEVE
One of the charms of psychotherapy is the frequent claim for permanent benefit since the pathogenic process has been quelled. Both Barlow et al. and Heimberg et al. indicate that the benefits of psychotherapy are better maintained than those of pharmacotherapy after treatment discontinuation. This implies a prophylactic lasting benefit. However, this may not be the case as indicated by Hollon et al. (1991) who states:
It remains possible that cognitive therapy’s apparent preventive effect represents the consequences of diffierential retention rather than any bona fide treatment effect. The typical follow-up study focuses on patients who both complete and respond to the respective modalities. Given the 20 - 40% attrition rates and 60 - 75% response rates associated with the respective interventions, this means that the samples entering follow-up might constitute only 35 - 60% of the sample initially assigned. If different types of patients are likely to either complete or respond to the respective modalities, the acute treatment period could act like a differential sieve, producing systematic differences in the sets of patients entering the follow-up from the different modalities. It is conceivable that the differences observed to date result not from any preventive effect attributable to cognitive therapy but rather from a greater propensity for patients at risk for relapes . . . to successfully complete pharmacotherapy than cognitive therapy.
Furthermore, studies often only superficially report longitudinal data. Brown and Barlow (1995) illustrate the difference between simple cross-sectional and longitudinal evaluations of fluctuating conditions. While cross-sectional data indicate that many subjects had improved during the follow-up period, the fact that some responders relapsed and other nonresponders improved was obscured until the data was analyzed longitudinally. This fluctuating pattern indicated that substantially fewer patients had a maintained high end state functioning.