Sample size/power calculations for repeated ordinal measurements in population pharmacodynamic experiments



Population pharmacodynamic experiments sometime involve repeated measurements of ordinal random variables at specific time points. Such longitudinal data presents a challenge during modelling due to correlation between measurements within an individual and often mixed-effects modelling approach may be used for the analysis. It is important that these studies are adequately powered by including an adequate number of subjects in order to detect a significant treatment effect. This paper describes a method for calculating sample size for repeated ordinal measurements in population pharmacodynamic experiments based on analysis by a mixed-effects modelling approach. The Wald test is used for testing the significance of treatment effects. This method is fast, simple and efficient. It can also be extended to account for differential allocation of subjects to the groups and unbalanced sampling designs between and within groups. The results obtained from two simulation studies using nonlinear mixed-effects modelling software (NONMEM) showed good agreement between the power obtained from simulation and nominal power used for sample size calculations.


Ordinal measurements Sample size Power Ordered categorical Cumulative logit Population pharmacodynamics Mixed-effects modelling Clinical trials 



The research was sponsored by the Centre for Applied Pharmacokinetic Research, School of Pharmacy and Pharmaceutical Sciences, The University of Manchester, Manchester, UK. (CAPKR is supported by the following consortium members; Eli Lilly, GlaxoSmithKline, Novartis, Pfizer and Servier).


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Copyright information

© Springer Science+Business Media, LLC 2009

Authors and Affiliations

  1. 1.Centre for Applied Pharmacokinetic ResearchThe University of ManchesterManchesterUK
  2. 2.School of Pharmacy and Pharmaceutical SciencesThe University of ManchesterManchesterUK

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