Intended for healthcare professionals

Letters Sample size calculations

An unorthodox approach to calculating sample size

BMJ 2012; 345 doi: (Published 02 October 2012) Cite this as: BMJ 2012;345:e6586
  1. Sara Muller, research associate1,
  2. Samantha L Hider, senior lecturer in rheumatology1,
  3. Toby Helliwell, National Institute for Health Research in-practice fellow1,
  4. Christian D Mallen, professor of general practice1
  1. 1Arthritis Research UK Primary Care Centre, Primary Care Sciences, Keele University, Keele ST5 5BG, UK
  1. s.muller{at}

Norman and colleagues raise an important issue—sample size calculations are at best an educated guess.1 While their suggestion might be helpful when an idea of sample size can be gleaned, in a recent study we had very little information at all.

The study was a primary care inception cohort of a rare and little researched condition. Initially, we …

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