A corrected formulation for marginal inference derived from two-part mixed models for longitudinal semi-continuous data. (October 2016)

• Record Type: Journal Article
• Authors: Tom, Brian DM; Su, Li; Farewell, Vernon T
• Is Part Of: Statistical methods in medical research. Volume 25:Number 5(2016:Oct.)
• Page Start: 2014
• Page End: 2020
• Publication Date: 2016-10
• Subjects: bridge distribution -- excess zeros -- longitudinal data -- random effects; Medicine -- Research -- Statistical methods -- Periodicals; Research -- Periodicals; Review Literature -- Periodicals; Statistics -- methods -- Periodicals; Médecine -- Recherche -- Méthodes statistiques -- Périodiques; 610.727
• DOI: 10.1177/0962280213509798
• Languages: English
• ISSNs: 0962-2802

Abstract

For semi-continuous data which are a mixture of true zeros and continuously distributed positive values, the use of two-part mixed models provides a convenient modelling framework. However, deriving population-averaged (marginal) effects from such models is not always straightforward. Su et al. presented a model that provided convenient estimation of marginal effects for the logistic component of the two-part model but the specification of marginal effects for the continuous part of the model presented in that paper was based on an incorrect formulation. We present a corrected formulation and additionally explore the use of the two-part model for inferences on the overall marginal mean, which may be of more practical relevance in our application and more generally.