title: A time-varying effect model for examining group differences in trajectories of zero-inflated count outcomes with applications in substance abuse research.
abstract: This study proposes a time-varying effect model for examining group differences in trajectories of zero-inflated count outcomes. The motivating example demonstrates that this zero-inflated Poisson model allows investigators to study group differences in different aspects of substance use (e.g., the probability of abstinence and the quantity of alcohol use) simultaneously. The simulation study shows that the accuracy of estimation of trajectory functions improves as the sample size increases; the accuracy under equal group sizes is only higher when the sample size is small (100). In terms of the performance of the hypothesis testing, the type I error rates are close to their corresponding significance levels under all settings. Furthermore, the power increases as the alternative hypothesis deviates more from the null hypothesis, and the rate of this increasing trend is higher when the sample size is larger. Moreover, the hypothesis test for the group difference in the zero component tends to be less powerful than the test for the group difference in the Poisson component. Copyright © 2016 John Wiley & Sons, Ltd.
authors.affiliation: Department of Statistics, Pennsylvania State University, University Park, PA 16802, U.S.A.
authors.forename: Songshan
authors.initials: S
authors.lastname: Yang
authors.affiliation: Department of Psychiatry & Addiction Research Center, University of Michigan, Ann Arbor, MI 48109, U.S.A.
authors.forename: James A
authors.initials: JA
authors.lastname: Cranford
authors.affiliation: Department of Psychiatry & Addiction Research Center, University of Michigan, Ann Arbor, MI 48109, U.S.A.
authors.forename: Jennifer M
authors.identifier: 0000-0003-3226-5057
authors.initials: JM
authors.lastname: Jester
authors.affiliation: Department of Statistics and the Methodology Center, Pennsylvania State University, University Park, PA 16802, U.S.A.
authors.forename: Runze
authors.identifier: 0000-0002-0154-2202
authors.initials: R
authors.lastname: Li
authors.affiliation: Department of Psychiatry & Addiction Research Center, University of Michigan, Ann Arbor, MI 48109, U.S.A.
authors.forename: Robert A
authors.initials: RA
authors.lastname: Zucker
authors.affiliation: Department of Health Behavior and Biological Sciences, University of Michigan, Ann Arbor, MI 48109, U.S.A.
authors.forename: Anne
authors.initials: A
authors.lastname: Buu
mesh_terms: D000293:Adolescent; D000428:Alcohol Drinking / Q000453:epidemiology; D000437:Alcoholism / Q000453:epidemiology / Q000209:etiology; D005260:Female; D006801:Humans; D008137:Longitudinal Studies; D008297:Male; D008824:Michigan; D015233:Models, Statistical; D016012:Poisson Distribution; D011336:Probability; D012307:Risk Factors; D018401:Sample Size; D012737:Sex Factors; D013223:Statistics as Topic / Q000379:methods; D019966:Substance-Related Disorders / Q000453:epidemiology* / Q000209:etiology; D013997:Time Factors; D055815:Young Adult
keywords: B-spline; longitudinal data; substance abuse; time-varying effect; zero-inflated Poisson model