MRI-based clinical trials in relapsing–remitting MS: new sample size calculations based on a longitudinal model

Multiple Sclerosis Journal, 04/27/2012

Sample sizes based on the longitudinal model are more realistic and often smaller than those previously reported using the negative binomial (NB) model.


  • The fit of the NB model in each arm of five MS clinical trials was assessed using Pearson’s chi-squared statistic.
  • Required sample sizes associated with various tests of treatment effect were estimated by simulating data from a new, longitudinal model for repeated lesion count data on individual patients.


  • Evidence (p < 0.05) against the NB model was found in at least one arm of four of the five trials.
  • If a trial is designed using this model but the resulting clinical data do not follow its assumptions then this trial can be seriously under-powered for assessing differences in mean lesion counts.

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