Review Shifting effects in randomised controlled trials of complex interventions: a new kind of performance bias? Gold C, Erkkila¨ J, Crawford MJ. Shifting effects in randomised controlled trials of complex interventions: a new kind of performance bias? Objective: Randomised controlled trials (RCTs) aim to provide unbiased estimates of treatment effects. However, the process of implementing trial procedures may have an impact on the performance of complex interventions that rely strongly on the intuition and confidence of therapists. We aimed to examine whether shifting effects over the recruitment period can be observed that might indicate such impact. Method: Three RCTs investigating music therapy vs. standard care were included. The intervention was performed by experienced therapists and based on established methods. We examined outcomes of participants graphically, analysed cumulative effects and tested for differences between first vs. later participants. We tested for potential confounding population shifts through multiple regression models. Results: Cumulative differences suggested trends over the recruitment period. Effect sizes tended to be less favourable among the first participants than later participants. In one study, effects even changed direction. Age, gender and baseline severity did not account for these shifting effects. Conclusion: Some trials of complex interventions have shifting effects over the recruitment period that cannot be explained by therapist experience or shifting demographics. Replication and further research should aim to find out which interventions and trial designs are most vulnerable to this new kind of performance bias. C. Gold 1 , J. Erkkilä 2 , M. J. Crawford 3 1 GAMUT, Uni Health, Uni Research, Bergen, Norway, 2 Finnish Centre of Excellence in Interdisciplinary Music Research, University of Jyväskylä, Finland and 3 Centre for Mental Health, Imperial College London, UK Key words: randomised controlled trial; psychotherapy; schizophrenia; depression Christian Gold, GAMUT, Uni Health, Uni Research, Lars Hilles gt. 3, 5015 Bergen, Norway. E-mail: christian.gold@uni.no Accepted for publication July 5, 2012 Summations • In trials of complex interventions, participants enrolled early may not experience the same level of benefit as later ones. • These shifting effects cannot be explained by demographic variables or therapist experience in general. • The impact of trial procedures on therapist performance should be considered in trials of complex interventions. Considerations • Findings are based on a small number of trials and on only one type of complex intervention. • Shifting effects may occur in some trials of complex interventions, but not in others. • Further research is needed to fully understand how trial procedures impact on therapist performance. Acta Psychiatr Scand 2012: 126: 307–314 All rights reserved DOI: 10.1111/j.1600-0447.2012.01922.x Ó 2012 John Wiley & Sons A/S ACTA PSYCHIATRICA SCANDINAVICA 307