title: Moderate concordance was found between case-only and parallel group designs in systematic comparison.
abstract: OBJECTIVE
  To empirically evaluate the concordance of effect estimates between case-only and parallel group designs and to identify predictors of discrepancies.
  
  
  METHODS
  MEDLINE and EMBASE databases were searched through June 31, 2013. Studies that used both a case-only (case crossover or self-controlled case series) and a parallel group design (cohort or case-control) were identified. Spearman correlation coefficient was used to evaluate the concordance between designs. Z-scores were used to assess whether differences in the effect estimates were common, using an absolute threshold value of 1.96. A prediction model was built to identify predictors of discrepancies.
  
  
  RESULTS
  The search identified 1,367 articles of which 53 were included for analysis. In total, 519 comparisons were made. The correlation coefficient between case-only vs. parallel group studies was 0.64 (P < .001). In 221 of the 519 comparisons (43%), the difference between both study designs was larger than the predetermined threshold. The following predictors of discrepancy were found: intermittent exposure, rare event, acute outcome, length of hazard period, type of case-only design, and sample size (C statistic of 0.783).
  
  
  CONCLUSIONS
  The concordance between effect estimates of case-only and parallel group designs is moderate. Such discrepancies could be predicted by failure to meet assumptions of case-only designs.
authors.affiliation: Unit of PharmacoEpidemiology & PharmacoEconomics, Department of Pharmacy, University of Groningen, Antonius Deusinglaan 1, 9713 AV Groningen, The Netherlands. Electronic address: k.b.pouwels@gmail.com.
authors.forename: Koen Bernardus
authors.initials: KB
authors.lastname: Pouwels
authors.affiliation: Unit of PharmacoEpidemiology & PharmacoEconomics, Department of Pharmacy, University of Groningen, Antonius Deusinglaan 1, 9713 AV Groningen, The Netherlands.
authors.forename: Bianca
authors.initials: B
authors.lastname: Mulder
authors.affiliation: Unit of PharmacoEpidemiology & PharmacoEconomics, Department of Pharmacy, University of Groningen, Antonius Deusinglaan 1, 9713 AV Groningen, The Netherlands; Department of Epidemiology, University Medical Center Groningen, University of Groningen, Hanzeplein 1, Postbus 30 001, 9700 RB Groningen, The Netherlands.
authors.forename: Eelko
authors.initials: E
authors.lastname: Hak
mesh_terms: D016022:Case-Control Studies; D000068456:Clinical Studies as Topic / Q000706:statistics & numerical data*; D015331:Cohort Studies; D018592:Cross-Over Studies; D016208:Databases, Factual / Q000706:statistics & numerical data*; D006801:Humans; D012107:Research Design
keywords: Case crossover; Case–control studies; Cohort studies; Crossover studies; Research design; Self-controlled case series