Describing the spread of data II

doi:10.1136/bmj.c1282

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Philip Sedgwick, senior lecturer in medical statistics

¹Centre for Medical and Healthcare Education, St George’s, University of London, London SW17 0RE

p.sedgwick{at}sgul.ac.uk

Last week’s question discussed sample standard deviation. For a variable such as body mass index (BMI), the sample standard deviation provides a measure of the average difference between the BMI of individuals in the sample and the sample mean.1

The example used last week was a cohort study: researchers investigated the association between slow walking speed and risk of death in older people.2 A cohort of 3208 French men and women aged over 65 years living in the community were recruited in 1999 and followed for an average of 5.1 years.

At baseline the cohort had a mean BMI of 25.6 kg m^-2 with a standard deviation of 4.0 kg m^-2. The …