title: Statistical power in parallel group point exposure studies with time-to-event outcomes: an empirical comparison of the performance of randomized controlled trials and the inverse probability of treatment weighting (IPTW) approach.
abstract: BACKGROUND
  Estimating statistical power is an important component of the design of both randomized controlled trials (RCTs) and observational studies. Methods for estimating statistical power in RCTs have been well described and can be implemented simply. In observational studies, statistical methods must be used to remove the effects of confounding that can occur due to non-random treatment assignment. Inverse probability of treatment weighting (IPTW) using the propensity score is an attractive method for estimating the effects of treatment using observational data. However, sample size and power calculations have not been adequately described for these methods.
  
  
  METHODS
  We used an extensive series of Monte Carlo simulations to compare the statistical power of an IPTW analysis of an observational study with time-to-event outcomes with that of an analysis of a similarly-structured RCT. We examined the impact of four factors on the statistical power function: number of observed events, prevalence of treatment, the marginal hazard ratio, and the strength of the treatment-selection process.
  
  
  RESULTS
  We found that, on average, an IPTW analysis had lower statistical power compared to an analysis of a similarly-structured RCT. The difference in statistical power increased as the magnitude of the treatment-selection model increased.
  
  
  CONCLUSIONS
  The statistical power of an IPTW analysis tended to be lower than the statistical power of a similarly-structured RCT.
authors.affiliation: Institute for Clinical Evaluative Sciences, G106, 2075 Bayview Avenue, M4N 3M5, Toronto, ON, Canada. peter.austin@ices.on.ca.|Institute of Health Management, Policy and Evaluation, University of Toronto, Toronto, Canada. peter.austin@ices.on.ca.|Schulich Heart Research Program, Sunnybrook Research Institute, Toronto, Canada. peter.austin@ices.on.ca.
authors.forename: Peter C
authors.initials: PC
authors.lastname: Austin
authors.affiliation: Clinical Epidemiology and Biostatistics Unit and Melbourne Children's Trial Centre, Murdoch Children's Research Institute, Royal Children's Hospital, Parkville, VIC, Australia. tibor.schuster@mcri.edu.au.|Department of Epidemiology, Biostatistics and Occupational Health, McGill University, Montreal, Canada. tibor.schuster@mcri.edu.au.|Department of Paediatrics, University of Melbourne, Melbourne, Australia. tibor.schuster@mcri.edu.au.
authors.forename: Tibor
authors.initials: T
authors.lastname: Schuster
authors.affiliation: Department of Epidemiology, Biostatistics and Occupational Health, McGill University, Montreal, Canada. Robert.platt@mcgill.ca.|Department of Pediatrics, McGill University, Montreal, Canada. Robert.platt@mcgill.ca.
authors.forename: Robert W
authors.initials: RW
authors.lastname: Platt
mesh_terms: D003627:Data Interpretation, Statistical; D006801:Humans; D009010:Monte Carlo Method; D064887:Observational Studies as Topic / Q000379:methods*; D057216:Propensity Score; D016016:Proportional Hazards Models; D016032:Randomized Controlled Trials as Topic / Q000379:methods*; D012107:Research Design; D016019:Survival Analysis