The sleeper effect: Artifact or phenomenon—A brief comment on Bell et al. (2013)

Flückiger, Christoph; Del Re, A. C.; Wampold, Bruce E. (2015). The sleeper effect: Artifact or phenomenon—A brief comment on Bell et al. (2013). Journal of consulting and clinical psychology, 83(2), pp. 438-442. American Psychological Association 10.1037/a0037220

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OBJECTIVE: Bell, Marcus, and Goodlad (2013) recently conducted a meta-analysis of randomized controlled additive trials and found that adding an additional component to an existing treatment vis-à-vis the existing treatment produced larger effect sizes on targeted outcomes at 6-months follow-up than at termination, an effect they labeled as a sleeper effect. One of the limitations with Bell et al.'s detection of the sleeper effect was that they did not conduct a statistical test of the size of the effect at follow-up versus termination. METHOD: To statistically test if the differences of effect sizes between the additive conditions and the control conditions at follow-up differed from those at termination, we used a restricted maximum-likelihood random-effect model with known variances to conduct a multilevel longitudinal meta-analysis (k = 30). RESULTS: Although the small effects at termination detected by Bell et al. were replicated (ds = 0.17-0.23), none of the analyses of growth from termination to follow-up produced statistically significant effects (ds < 0.08; p > .20), and when asymmetry was considered using trim-and-fill procedure or the studies after 2000 were analyzed, magnitude of the sleeper effect was negligible (d = 0.00). CONCLUSION: There is no empirical evidence to support the sleeper effect.

Item Type:

Journal Article (Review Article)


07 Faculty of Human Sciences > Institute of Psychology > Clinical Psychology and Psychotherapy
07 Faculty of Human Sciences > Institute of Psychology

UniBE Contributor:

Flückiger, Christoph


100 Philosophy > 150 Psychology
600 Technology > 610 Medicine & health




American Psychological Association




Adriana Biaggi

Date Deposited:

14 Apr 2015 16:29

Last Modified:

01 Jul 2015 16:47

Publisher DOI:



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