title: Absolute risks rather than incidence rates should be used to estimate the number needed to treat from time-to-event data.
abstract: BACKGROUND
  When estimating the number needed to treat (NNT) from randomized controlled trials (RCTs) with time-to-event outcomes, varying follow-up times have to be considered. Two methods have been proposed, namely (1) inverting risk differences estimated by survival time methods (RD approach) and (2) inverting incidence differences (ID approach).
  
  
  METHODS
  A simulation study was conducted to compare the RD and the ID approaches regarding bias and coverage probability (CP) considering various distributions, baseline risks, effect sizes, and sample sizes. Additionally, the two approaches were compared by using two real data examples.
  
  
  RESULTS
  The RD approach showed good estimation and coverage properties with only a few exceptions in the case of small sample sizes and small effect sizes. The ID approach showed considerable bias and low CPs in most of the considered data situations.
  
  
  CONCLUSIONS
  Absolute risks estimated by means of survival time methods rather than incidence rates should be used to estimate NNTs in RCTs with time-to-event outcomes.
authors.affiliation: Department of Medical Biometry, Institute for Quality and Efficiency in Health Care, Im Mediapark 8, D-50670, Cologne, Germany. Ralf@rbsd.de
authors.forename: Ralf
authors.initials: R
authors.lastname: Bender
authors.forename: Mandy
authors.initials: M
authors.lastname: Kromp
authors.forename: Corinna
authors.initials: C
authors.lastname: Kiefer
authors.forename: Sibylle
authors.initials: S
authors.lastname: Sturtz
mesh_terms: D015982:Bias; D003198:Computer Simulation; D015340:Epidemiologic Research Design; D006801:Humans; D015994:Incidence; D053208:Kaplan-Meier Estimate; D061366:Numbers Needed To Treat; D011336:Probability; D016032:Randomized Controlled Trials as Topic; D013997:Time Factors
keywords: Incidence rate; Kaplan–Meier curve; Number needed to treat; Randomized controlled trials; Simulation; Time-to-event data