Statistical Question

What is a P value?

BMJ 2012; 345 doi: http://dx.doi.org/10.1136/bmj.e7767 (Published 21 November 2012)
Cite this as: BMJ 2012;345:e7767

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  1. Philip Sedgwick, reader in medical statistics and medical education
  1. 1Centre for Medical and Healthcare Education, St George’s, University of London, Tooting, London, UK
  1. p.sedgwick{at}sgul.ac.uk

Researchers investigated whether rapid rather than standard intravenous rehydration resulted in improved hydration and clinical outcomes when administered to children with gastroenteritis. Treatments were compared in a parallel randomised controlled trial. Children were recruited if aged 3 months to 11 years, had a diagnosis of dehydration secondary to gastroenteritis, had not responded to oral rehydration, and had been prescribed intravenous rehydration. Intervention was rapid (60 mL/kg) or standard (20 mL/kg) rehydration with 0.9% saline over an hour.1

The primary outcome was the proportion of children with clinical rehydration, assessed on a validated scale, within two hours of start of treatment. In total, 226 children were recruited, of whom 114 were randomised to rapid and 112 to standard rehydration. The proportion of children rehydrated at two hours was higher in the rapid rehydration group, although the difference was not significant (41/114 (36%) v 33/112 (29.5%); P=0.32).

Which one of the following statements best describes the P value?

  • a) It is the probability that the null hypothesis is true.

  • b) It is the probability that the alternative hypothesis is true.

  • c) It is the probability of obtaining the observed difference in the outcome measure, or a larger one, given that no difference exists between treatments in the population.

  • d) It is the probability that the observed difference in the outcome measure was due to random chance.

Answers

Statement c best describes the P value.

The trial investigated whether rapid rather than standard intravenous rehydration resulted in improved rehydration and clinical outcomes when …

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