Endgames Statistical Quiz

Sample size calculations I

BMJ 2010; 340 doi: http://dx.doi.org/10.1136/bmj.c3104 (Published 16 June 2010) Cite this as: BMJ 2010;340:c3104
  1. Philip Sedgwick, senior lecturer in medical statistics
  1. 1Centre for Medical and Healthcare Education, St George’s, University of London, London SW17 0RE
  1. p.sedgwick{at}sgul.ac.uk

    A randomised, double blind, placebo controlled trial investigated whether fluvastatin reduced major adverse cardiac events in patients who had undergone successful percutaneous coronary intervention (with or without stenting). The primary outcome was the occurrence of major adverse cardiac events—defined as cardiac death, non-fatal myocardial infarction, or a reintervention procedure—within three years.1

    To calculate the sample size needed to compare fluvastatin with placebo, it was assumed that the proportion of patients having major adverse cardiac events at three years without treatment would be 25%. For fluvastatin to be considered clinically superior to placebo, it would be necessary to demonstrate a relative improvement of 25%, with only 18.75% of patients having major adverse cardiac events at three years. To do so, a total sample size of 1828 patients (914 in each treatment arm) would be required to achieve 90% power using a two sided hypothesis test and critical level of significance of 0.05. A total of 1677 patients were subsequently recruited to the trial.

    Which of the following, if any, are true?

    • a) The specified difference in major adverse cardiac events between fluvastatin and placebo is called the smallest …

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