title: Empirical evidence of bias in infertility research: overestimation of treatment effect in crossover trials using pregnancy as the outcome measure.
abstract: OBJECTIVE
  To determine whether crossover trials with simple pooling of data over different study periods leads to a different estimate of treatment effect compared with parallel group trials in infertility research using pregnancy as the outcome measure.
  
  
  METHODS
  An observational study using nine overviews that included trials with both crossover and parallel group designs. These overviews comprised 17 crossover and 17 parallel group trials. In total, there were 5,291 outcomes including 775 pregnancies. The association between study design and treatment effect estimate was analyzed using multiple logistic regression, controlling for differences in the therapeutic interventions and variations in the methodological quality of the trials.
  
  
  METHODS
  Infertile patients in an academic research environment.
  
  
  METHODS
  Infertile patients undergoing treatment efficacy evaluation in controlled trials.
  
  
  METHODS
  Random allocation to a variety of treatments including clomiphene citrate, hCG, IUI, tamoxifen, and bromocriptine.
  
  
  METHODS
  Estimate of bias between study designs, based on the interaction of study design and treatment in the logistic regression model.
  
  
  RESULTS
  Crossover trials produced a larger average estimate of treatment effect compared with trials with a parallel group design, overestimating the odds ratio by 74% (95% confidence interval, 2% to 197%).
  
  
  CONCLUSIONS
  The use of a crossover design for evaluating infertility treatments with outcomes that prevent patients from completing later phases of the trial should be avoided because it leads to exaggerated estimates of treatment effect and may result in erroneous inferences and clinical decisions. Furthermore, the type of study design should be taken into account when assessing the methodological quality of therapy trials in infertility.
authors.affiliation: Department of Clinical Epidemiology and Biostatistics, McMaster University, Hamilton, Ontario, Canada.
authors.forename: K S
authors.initials: KS
authors.lastname: Khan
authors.forename: S
authors.initials: S
authors.lastname: Daya
authors.forename: J A
authors.initials: JA
authors.lastname: Collins
authors.forename: S D
authors.initials: SD
authors.lastname: Walter
mesh_terms: D015982:Bias; D018592:Cross-Over Studies; D005260:Female; D006801:Humans; D007246:Infertility / Q000628:therapy*; D011247:Pregnancy; D012107:Research Design / Q000706:statistics & numerical data*