Endgames Statistical Question

Skewed distributions

BMJ 2010; 341 doi: http://dx.doi.org/10.1136/bmj.c6276 (Published 11 November 2010) Cite this as: BMJ 2010;341:c6276
  1. Philip Sedgwick, senior lecturer in medical statistics
  1. 1Section of Medical and Healthcare Education, St George’s, University of London, Tooting, London, UK
  1. p.sedgwick{at}sgul.ac.uk

Researchers investigated the trend in sperm counts between 1940 and 1990.1 They measured sperm concentrations in 235 men presenting for stimulated in vitro fertilisation (fig 1) and compared these measurements with reference ranges for the 50 year period.

Fig 1 Distribution of sperm concentration in 235 men presenting for stimulated in vitro fertilisation

A separate study investigated whether management of depression in UK general practice varied with severity.2 It measured severity of depression using the self completed nine item patient health questionnaire (PHQ-9). Patients obtained a score between zero and 27, with high scores representing greater severity. A total of 1658 depressed patients were assessed (fig 2 ).

Fig 2 Distribution of depression severity for 1658 patients in general practice as assessed by the PHQ-9 questionnaire

Which of the following statements, if any, are true?

  • a) The distribution of the sperm …

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