text: Competing interests
  None declared.
  This article is the third in an ongoing, educational review series on medical statistics in critical care. Previous articles have covered 'presenting and summarising data' [3] and 'samples and populations' [4]. Future topics to be covered include power calculations, comparison of means, comparison of proportions, and analysis of survival data to name but a few. If there is a medical statistics topic you would like explained contact us on editorial@ccforum.com .
  