title: High statistical heterogeneity is more frequent in meta-analysis of continuous than binary outcomes.
abstract: OBJECTIVE
  We compared the distribution of heterogeneity in meta-analyses of binary and continuous outcomes.
  
  
  METHODS
  We searched citations in MEDLINE and Cochrane databases for meta-analyses of randomized trials published in 2012 that reported a measure of heterogeneity of either binary or continuous outcomes. Two reviewers independently performed eligibility screening and data abstraction. We evaluated the distribution of I(2) in meta-analyses of binary and continuous outcomes and explored hypotheses explaining the difference in distributions.
  
  
  RESULTS
  After full-text screening, we selected 671 meta-analyses evaluating 557 binary and 352 continuous outcomes. Heterogeneity as assessed by I(2) proved higher in continuous than in binary outcomes: the proportion of continuous and binary outcomes reporting an I(2) of 0% was 34% vs. 52%, respectively, and reporting an I(2) of 60-100% was 39% vs. 14%. In continuous but not binary outcomes, I(2) increased with larger number of studies included in a meta-analysis. Increased precision and sample size do not explain the larger I(2) found in meta-analyses of continuous outcomes with a larger number of studies.
  
  
  CONCLUSIONS
  Meta-analyses evaluating continuous outcomes showed substantially higher I(2) than meta-analyses of binary outcomes. Results suggest differing standards for interpreting I(2) in continuous vs. binary outcomes may be appropriate.
authors.affiliation: Heart Failure and Transplant Program, Toronto General Hospital, University Health Network, 585 University Ave, 11c PMB 153, Toronto, Ontario M5G 2N2, Canada. Electronic address: Carolina.alba@uhn.ca.
authors.forename: Ana C
authors.initials: AC
authors.lastname: Alba
authors.affiliation: Clinical Epidemiology and Biostatistics, McMaster University, 1280 Main Street West, Hamilton, Ontario L8S 4K1, Canada.
authors.forename: Paul E
authors.initials: PE
authors.lastname: Alexander
authors.affiliation: Heart Failure and Transplant Program, Toronto General Hospital, University Health Network, 585 University Ave, 11c PMB 153, Toronto, Ontario M5G 2N2, Canada.
authors.forename: Joanne
authors.initials: J
authors.lastname: Chang
authors.affiliation: Heart Failure and Transplant Program, Toronto General Hospital, University Health Network, 585 University Ave, 11c PMB 153, Toronto, Ontario M5G 2N2, Canada.
authors.forename: John
authors.initials: J
authors.lastname: MacIsaac
authors.affiliation: Heart Failure and Transplant Program, Toronto General Hospital, University Health Network, 585 University Ave, 11c PMB 153, Toronto, Ontario M5G 2N2, Canada.
authors.forename: Samantha
authors.initials: S
authors.lastname: DeFry
authors.affiliation: Clinical Epidemiology and Biostatistics, McMaster University, 1280 Main Street West, Hamilton, Ontario L8S 4K1, Canada.
authors.forename: Gordon H
authors.initials: GH
authors.lastname: Guyatt
mesh_terms: D004812:Epidemiologic Methods; D006801:Humans; D015201:Meta-Analysis as Topic; D015233:Models, Statistical; D016032:Randomized Controlled Trials as Topic
keywords: Binary outcomes; Continuous outcomes; Heterogeneity; I(2); Meta-analysis; Number of studies