title: Modeling repeated count measures with excess zeros in an epidemiological study.
abstract: OBJECTIVE
  Highly skewed count data with excess zeros challenge the application of conventional statistical methods. Additional problems arise from repeated zero-inflated measures. Longitudinal zero-inflated Poisson (ZIP-mixed) models are mixtures of logistic and Poisson models that accommodate excess zeros and repeated counts. We compared a ZIP-mixed model with traditional Poisson and negative binomial models using data on problems with female condom use reported by women at high risk of sexually transmitted diseases.
  
  
  METHODS
  The follow-up experience of this cohort represents a mixture of "perfect use" (no opportunity to report problems), represented by the structural zeros, and use experience that bears the risk of condom use problems, represented by a Poisson distribution.
  
  
  RESULTS
  The ZIP-mixed model provided better fit and richer results than other models. The odds of being in the zero problem category increased with age (odds ratio [OR] = 1.1 per additional year, 95% confidence interval [CI]: 1.0-1.3) and with follow-up (OR = 3.0 per additional month, 95% CI: 1.4-6.0).The nonzero problem rate was lower among women who believed in the benefits of condom use (rate ratio [RR] = 0.9, 95% CI: 0.7-1.0) and had no sexually transmitted diseases at baseline (RR = 0.7, 95% CI: 0.6-0.9), and it decreased during follow-up (RR = 0.8 per additional month, 95% CI: 0.7-0.9).
  
  
  CONCLUSIONS
  Using ZIP-mixed model provided further insights into the determinants of condom failure.
authors.affiliation: Division of Biostatistics and Epidemiology, Cincinnati Children's Hospital Medical Center, Cincinnati, OH. Electronic address: Resmi.Gupta@cchmc.org.
authors.forename: Resmi
authors.initials: R
authors.lastname: Gupta
authors.affiliation: Division of Biostatistics and Epidemiology, Cincinnati Children's Hospital Medical Center, Cincinnati, OH.
authors.forename: Rhonda D
authors.initials: RD
authors.lastname: Szczesniak
authors.affiliation: Division of Biostatistics and Epidemiology, Cincinnati Children's Hospital Medical Center, Cincinnati, OH.
authors.forename: Maurizio
authors.initials: M
authors.lastname: Macaluso
mesh_terms: D019111:Single-Use Internal Condom / Q000706:statistics & numerical data*; D016021:Epidemiologic Studies; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D015233:Models, Statistical; D016017:Odds Ratio; D016012:Poisson Distribution; D012044:Regression Analysis; D021841:Safe Sex / Q000706:statistics & numerical data*; D012747:Sexual Partners; D055815:Young Adult
keywords: Generalized linear model; Longitudinal zero-inflated Poisson; Zero-inflated Poisson mixed