JSM 2004 - Toronto

Abstract #301898

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Activity Number: 56
Type: Contributed
Date/Time: Sunday, August 8, 2004 : 4:00 PM to 5:50 PM
Sponsor: Section on Statistics in Epidemiology
Abstract - #301898
Title: Modeling Variations over Time Using Multilevel Procedures: An Illustration to Model Counts and Number of Protected Coital Acts
Author(s): Emelita L. Wong*+
Companies: Family Health International
Address: PO Box 13950, Research Triangle, NC, 27709,
Keywords: multilevel modeling ; protected coital acts
Abstract:

To describe patterns of responses over time, regression lines where the explanatory variables consist of time intervals may be fitted. Within a multilevel model framework, such regression line has two components, namely: (1) fixed-effects components, which present mean outcome at time point coded as zero and an overall trend over time, and (2) random effects components, the first set of which measures how the overall mean varies from subject-to-subject and from time point to time point, while the second set measures how the trend (slope) of responses over time varies from subject-to subject and joint variation of the overall mean and overall trends. The data for the illustrative analysis consist of self-reports of number and proportions of protected coital acts in 5,241 occasions, reported monthly for six months during trial participation, and at approximately 14 months after the trial among 964 Cameroonian women. While the fixed effect estimate of trends indicates overall decrease in proportion of protected coital acts over time, the random effects estimates differentiate three types of trends over time: decreasing, no change, and increasing.

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