A comparison of different ways of including baseline counts in negative binomial models for data from falls prevention trials
© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Reason for embargo
A common design for a falls prevention trial is to assess falling at baseline, randomize participants into an intervention or control group, and ask them to record the number of falls they experience during a follow-up period of time. This paper addresses how best to include the baseline count in the analysis of the follow-up count of falls in Negative Binomial (NB) regression. We examine the performance of various approaches in simulated datasets where both counts are generated from a mixed Poisson distribution with shared random subject effect. Including the baseline count after log-transformation as a regressor in NB regression (NB-logged) or as an offset (NB-offset) resulted in greater power than including the untransformed baseline count (NB-unlogged). Cook and Wei's Conditional Negative Binomial (CNB) model replicates the underlying process generating the data. In our motivating dataset, a statistically significant intervention effect resulted from the NB-logged, NB-offset and CNB models, but not from NB-unlogged, and large, outlying baseline counts were overly influential in NB-unlogged but not in NB-logged. We conclude that there is little to lose by including the log-transformed baseline count in standard NB regression compared to CNB for moderate to larger sized datasets.
Funding Information: National Institute for Health Research, Grant no. RDA/02/06/41; Care South West Peninsula
This is the author accepted manuscript. The final version is available from Wiley-VCH Verlag via the DOI in this record.
Published online 25 October 2017