Intended for healthcare professionals

Endgames Statistical Question

Parametric v non-parametric statistical tests

BMJ 2012; 344 doi: (Published 14 March 2012) Cite this as: BMJ 2012;344:e1753
  1. Philip Sedgwick, senior lecturer in medical statistics
  1. 1Centre for Medical and Healthcare Education, St George’s, University of London, Tooting, London, UK
  1. p.sedgwick{at}

Researchers investigated five year mortality in patients with chronic heart failure by comparing those with impaired left ventricular function (n=359) with those with preserved function (n=163).1 A prospective cohort study design was used, with patients enrolled if they had had stable symptomatic chronic heart failure for at least three months.

Characteristics of patients measured at recruitment included age and heart rate. Patients with preserved function were a similar age to those with impaired function (62.5 (standard deviation 10.7) v 62.3 (9.10) years; independent samples t test: P=0.80). Those with preserved function had a lower median heart rate (69 (interquartile range 63-82) v 76 (66-89) beats/min; Mann-Whitney U test P<0.001). Five year mortality was significantly greater in patients with impaired left ventricular systolic function (41.5% v 25.2%; P<0.001).

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

  • a) The independent samples t test is a parametric test.

  • b) The use of the independent samples t test assumed that age was normally distributed for the patient groups in the population.

  • c) The Mann-Whitney test is a non-parametric test.

  • d) The use of the Mann-Whitney U …

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