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Updated: May 6, 2026

Development of an Individual-Tree Basal Area Increment Model using a Linear Mixed-Effects Approach
Published on: July 3, 2020
A corrected formulation for marginal inference derived from two-part mixed models for longitudinal semi-continuous
Brian Dm Tom1, Li Su2, Vernon T Farewell2
1Medical Research Council Biostatistics Unit, Institute of Public Health, Cambridge, UK brian.tom@mrc-bsu.cam.ac.uk.
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.
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