Beware the "Texas sharp shooter" in rate ratios of progression

doi:10.1136/bmj.39136.475313.FA

BMJ 2007;334:440 (3 March), doi:10.1136/bmj.39136.475313.FA

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Randomised trials

Beware the "Texas sharp shooter" in rate ratios of progression

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Glasziou et al's method of calculating rate ratios of progression(stable unchanging condition before v change shortly after theintervention) is appealing,¹ but we need to be wary of a "Texassharp shooter" effect. This effect is usually associated inepidemiology with the problem of interpreting apparent clustersof disease in space, where the geographical unit of analysismay have been chosen post hoc so as to maximise the apparentdensity of cases (the sharp shooter metaphor comes from a jokeabout a Texan firing bullets into the wall of a barn and thendrawing the targets around the bullet holes to show his shootingprowess).

An analogous problem may occur when calculating rate ratiosin the manner described in this article, although the sharpshooting is in time, not space. In the mother's kiss, the timeperiod used is 10 s, which gives a rate ratio of progressionof . . . [Full text of this article]

Anna C Goodman

London School of Hygiene and Tropical Medicine, London WC1E 7HT

anna.goodman@lshtm.ac.uk

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