title: N-of-1 trials in the medical literature: a systematic review.
abstract: BACKGROUND
  N-of-1 trials (multiple crossover studies conducted in single individuals) may be ideal for determining individual treatment effects and as a tool to estimate heterogeneity of treatment effects (HTE) in a population. However, comprehensive data on n-of-1 trial methodology and analysis is lacking. We performed this study to describe n-of-1 trial characteristics, examine treatment changes resulting from n-of-1 trial participation, and to determine if trial reporting is adequate for estimating HTE.
  
  
  METHODS
  We undertook a systematic review of n-of-1 trials published between 1985 and December 2010. Included trials were those having individual treatment episodes as the unit of randomization and reporting individual-specific treatment effects. We abstracted trial characteristics, treatment change information, and analytic methods.
  
  
  RESULTS
  We included 108 trials reporting on 2154 participants. Approximately half (49%) of the trials used a statistical cutoff to determine a superior treatment, whereas the remainder used a graphical comparison (25%) or a clinical significance cutoff (20%). Sixty-seven trials, reporting on 488 people, provided treatment change information: 54% of participants had subsequent treatment decisions consistent with the results of the trial, 8% had decisions inconsistent with trial results, and 38% had ambiguous results. Less than half of the trials (45%) reported adequate information to facilitate the calculation of HTE.
  
  
  CONCLUSIONS
  N-of-1 trials are a useful tool for enhancing therapeutic precision in a range of conditions and should be conducted more often. To facilitate future meta-analysis, and the estimation of HTE, researchers reporting n-of-1 trial results should clearly describe individual data.
authors.affiliation: Center for Clinical Epidemiology and Biostatistics and Department of Biostatistics and Epidemiology, University of Pennsylvania School of Medicine, Philadelphia, PA 19104-6021, USA. gabler@upenn.edu
authors.forename: Nicole B
authors.initials: NB
authors.lastname: Gabler
authors.forename: Naihua
authors.initials: N
authors.lastname: Duan
authors.forename: Sunita
authors.initials: S
authors.lastname: Vohra
authors.forename: Richard L
authors.initials: RL
authors.lastname: Kravitz
mesh_terms: D002986:Clinical Trials as Topic; D018592:Cross-Over Studies; D006801:Humans; D015233:Models, Statistical; D012107:Research Design