BMJ  2005;330:1448 (18 June), doi:10.1136/bmj.330.7505.1448-a

Letter

Was Rodney Ledward a statistical outlier?

Initial investigations of outliers must be chosen carefully

The first 150 words of the full text of this article appear below.

EDITOR—In their paper investigating whether Rodney Ledward was a statistical outlier Harley et al do not define what they mean by statistical outlier.¹ If it is a person whose outlying position in the distribution is almost certainly not due to chance, then they have failed. The numbers outside the 95% confidence intervals are as anticipated from the size of the population, and therefore they are simple outliers. They probably have refined the analysis so that those at the extremes are more likely to be aberrant performers, but they have not shown statistically that this is so. To do so, it would be necessary to show that a disproportionate number were outside the expected confidence intervals. This might be done by demonstrating two populations or possibly by recalculating the confidence intervals, using only those between, say, the 20th and 80th centiles and looking for wide outliers (using 0.1% intervals), . . . [Full text of this article]

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Aldbrough St John, Richmond, North Yorkshire DL117TP ck-r.connolly@medix-uk.com

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