title: Weighting condom use data to account for nonignorable cluster size.
abstract: OBJECTIVE
  We examined the impact of weighting the generalized estimating equation (GEE) by the inverse of the number of sex acts on the magnitude of association for factors predictive of recent condom use.
  
  
  METHODS
  Data were analyzed from a cross-sectional survey on condom use reported during vaginal intercourse during the past year among male students attending two Georgia universities. The usual GEE model was fit to the data predicting the binary act-specific response indicating whether a condom was used. A second cluster-weighted GEE model (i.e., weighting the GEE score equation by the inverse of the number of sex acts) was also fit to predict condom use.
  
  
  RESULTS
  Study participants who engaged in a greater frequency of sex acts were less likely to report condom use, resulting in nonignorable cluster-size data. The GEE analysis weighted by sex act (usual GEE) and the GEE analysis weighted by study subject (cluster-weighted GEE) produced different estimates of the association between the covariates and condom use in last year. For example, the cluster-weighted GEE analysis resulted in a marginally significant relationship between age and condom use (odds ratio of 0.49 with 95% confidence interval (0.23-1.03) for older versus younger participants) versus a nonsignificant relationship with the usual GEE model (odds ratio of 0.67 with a 95% confidence interval of 0.28-1.60).
  
  
  CONCLUSIONS
  The two ways of weighting the GEE score equation, by the sex act or by the respondent, may produce different results and a different interpretation of the parameters in the presence of nonignorable cluster size.
authors.affiliation: National Center for Infectious Diseases, Division of Parasitic Diseases, Centers for Disease Control and Prevention (CDC), Atlanta, GA 30341, USA. jow5@cdc.gov
authors.forename: John M
authors.initials: JM
authors.lastname: Williamson
authors.forename: Hae-Young
authors.initials: HY
authors.lastname: Kim
authors.forename: Lee
authors.initials: L
authors.lastname: Warner
mesh_terms: D000293:Adolescent; D000328:Adult; D001741:Black or African American / Q000523:psychology / Q000706:statistics & numerical data; D016000:Cluster Analysis; D003075:Coitus; D017280:Condoms / Q000706:statistics & numerical data*; D003430:Cross-Sectional Studies; D005845:Georgia / Q000453:epidemiology; D006801:Humans; D008297:Male; D012725:Sexual Behavior / Q000208:ethnology; D012749:Sexually Transmitted Diseases / Q000453:epidemiology / Q000208:ethnology / Q000517:prevention & control*; D013334:Students / Q000523:psychology / Q000706:statistics & numerical data; D014495:Universities