title: A corrected formulation for marginal inference derived from two-part mixed models for longitudinal semi-continuous data.
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.
authors.affiliation: Medical Research Council Biostatistics Unit, Institute of Public Health, Cambridge, UK brian.tom@mrc-bsu.cam.ac.uk.
authors.forename: Brian Dm
authors.initials: BD
authors.lastname: Tom
authors.affiliation: Medical Research Council Biostatistics Unit, Institute of Public Health, Cambridge, UK.
authors.forename: Li
authors.initials: L
authors.lastname: Su
authors.affiliation: Medical Research Council Biostatistics Unit, Institute of Public Health, Cambridge, UK.
authors.forename: Vernon T
authors.initials: VT
authors.lastname: Farewell
mesh_terms: D015535:Arthritis, Psoriatic / Q000235:genetics / Q000473:pathology / Q000503:physiopathology; D003627:Data Interpretation, Statistical; D005260:Female; D015796:HLA-B27 Antigen / Q000235:genetics; D006801:Humans; D016015:Logistic Models; D008137:Longitudinal Studies; D015233:Models, Statistical
chemical_list: D015796:HLA-B27 Antigen
keywords: bridge distribution; excess zeros; longitudinal data; random effects