title: Empirical comparison of subgroup effects in conventional and individual patient data meta-analyses.
abstract: OBJECTIVE
  Individual patient data (IPD) meta-analyses have been proposed as a major improvement in meta-analytic methods to study subgroup effects. Subgroup effects of conventional and IPD meta-analyses using identical data have not been compared. Our objective is to compare such subgroup effects using the data of six trials (n = 1,643) on the effectiveness of antibiotics in children with acute otitis media (AOM).
  
  
  METHODS
  Effects (relative risks, risk differences [RD], and their confidence intervals [CI]) of antibiotics in subgroups of children with AOM resulting from (i) conventional meta-analysis using summary statistics derived from published data (CMA), (ii) two-stage approach to IPD meta-analysis using summary statistics derived from IPD (IPDMA-2), and (iii) one-stage approach to IPD meta-analysis where IPD is pooled into a single data set (IPDMA-1) were compared.
  
  
  RESULTS
  In the conventional meta-analysis, only two of the six studies were included, because only these reported on relevant subgroup effects. The conventional meta-analysis showed larger (age < 2 years) or smaller (age > or = 2 years) subgroup effects and wider CIs than both IPD meta-analyses (age < 2 years: RDCMA -21 percent, RDIPDMA-1 -16 percent, RDIPDMA-2 -15 percent; age > or =2 years: RDCMA -5 percent, RDIPDMA-1 -11 percent, RDIPDMA-2 -11 percent). The most important reason for these discrepant results is that the two studies included in the conventional meta-analysis reported outcomes that were different both from each other and from the IPD meta-analyses.
  
  
  CONCLUSIONS
  This empirical example shows that conventional meta-analyses do not allow proper subgroup analyses, whereas IPD meta-analyses produce more accurate subgroup effects. We also found no differences between the one- and two-stage meta-analytic approaches.
authors.affiliation: University Medical Center Utrecht, Utrecht, The Netherlands. l.koopman@umcutrecht.nl
authors.forename: Laura
authors.initials: L
authors.lastname: Koopman
authors.forename: Geert J M G
authors.initials: GJ
authors.lastname: van der Heijden
authors.forename: Arno W
authors.initials: AW
authors.lastname: Hoes
authors.forename: Diederick E
authors.initials: DE
authors.lastname: Grobbee
authors.forename: Maroeska M
authors.initials: MM
authors.lastname: Rovers
mesh_terms: D000900:Anti-Bacterial Agents / Q000627:therapeutic use; D002648:Child; D002675:Child, Preschool; D003627:Data Interpretation, Statistical; D036262:Empirical Research; D006801:Humans; D007223:Infant; D015201:Meta-Analysis as Topic; D017063:Outcome Assessment, Health Care / Q000379:methods*
chemical_list: D000900:Anti-Bacterial Agents