title: The sleeper effect: Artifact or phenomenon-A brief comment on Bell et al. (2013).
abstract: 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.
  
  
  METHODS
  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).
  
  
  CONCLUSIONS
  There is no empirical evidence to support the sleeper effect. (PsycINFO Database Record
authors.affiliation: Department of Psychology.
authors.forename: Christoph
authors.initials: C
authors.lastname: Flückiger
authors.affiliation: Veterans Affairs Palo Alto Health Care System.
authors.forename: A C
authors.initials: AC
authors.lastname: Del Re
authors.affiliation: Department of Counseling Psychology, University of Wisconsin-Madison.
authors.forename: Bruce E
authors.initials: BE
authors.lastname: Wampold
mesh_terms: D006801:Humans; D008960:Models, Psychological; D016896:Treatment Outcome