title: Estimating risk difference from relative association measures in meta-analysis can infrequently pose interpretational challenges.
abstract: OBJECTIVE
  Risk difference (RD) is often estimated from relative association measures generated by meta-analysis and a particular group's baseline risk. We describe a problematic situation in using this approach.
  
  
  METHODS
  We encountered a meta-analysis in which a confidence interval (CI) of relative risk (RR) overlapped 1.0; the point estimate favored treatment A, but when we used RR and median baseline risk to calculate a CI for RD, a greater portion of the CI favored treatment B (a result that some may find counterintuitive). We then calculated 10 different RDs from recently published meta-analyses in outcomes in which CIs of RR crossed 1.0 using three methods: estimation from RR, estimation from the odds ratio, and pooling RDs across trials.
  
  
  RESULTS
  When RD is estimated from relative measures, the counterintuitive result occurred in 2 of 10 instances. This discordance of interpretation is because of the logarithmic transformation that makes CIs of relative measures asymmetric around their point estimates.
  
  
  CONCLUSIONS
  When RD is estimated from relative association measures that are nonsignificant and this counterintuitive situation occurs, it may be more appropriate to pool RD across studies. Pooling is particularly valid when baseline risks across studies are homogeneous.
authors.affiliation: The Knowledge and Encounter Research Unit, Mayo Clinic, Rochester, MN 55905, USA. murad.mohammad@mayo.edu
authors.forename: Mohammad Hassan
authors.initials: MH
authors.lastname: Murad
authors.forename: Victor M
authors.initials: VM
authors.lastname: Montori
authors.forename: Stephen D
authors.initials: SD
authors.lastname: Walter
authors.forename: Gordon H
authors.initials: GH
authors.lastname: Guyatt
mesh_terms: D002340:Carotid Artery Diseases / Q000401:mortality / Q000601:surgery; D003627:Data Interpretation, Statistical; D004691:Endarterectomy / Q000009:adverse effects / Q000401:mortality; D006801:Humans; D015201:Meta-Analysis as Topic; D012196:Review Literature as Topic; D018570:Risk Assessment / Q000379:methods*; D015607:Stents / Q000009:adverse effects; D016896:Treatment Outcome