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

Letters

Randomised trials

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

Glasziou et al's method of calculating rate ratios of progression (stable unchanging condition before v change shortly after the intervention) is appealing,¹ but we need to be wary of a "Texas sharp shooter" effect. This effect is usually associated in epidemiology with the problem of interpreting apparent clusters of disease in space, where the geographical unit of analysis may have been chosen post hoc so as to maximise the apparent density of cases (the sharp shooter metaphor comes from a joke about a Texan firing bullets into the wall of a barn and then drawing the targets around the bullet holes to show his shooting prowess).

An analogous problem may occur when calculating rate ratios in the manner described in this article, although the sharp shooting is in time, not space. In the mother's kiss, the time period used is 10 s, which gives a rate ratio of progression of 1440. Perhaps, however, the bead dislodged after only 8 s, a rate ratio of 1440/0.8=1800. Alternatively, if the bead had taken 15 s to dislodge, the doctor, nurse, and mother might still reasonably have felt that they should take the credit for the happy outcome. The point is that one needs to make an a priori decision about the post intervention time frame you will use—presumably based on the maximum length of time after the event during which, if improvement occurs, you are prepared to attribute it to your intervention.

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

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Competing interests: None declared.

References

1. Glasziou P, Chalmers I, Rawlins M, McCulloch P. When are randomised trials unnecessary? Picking signal from noise. BMJ 2007;334:349-51. (17 February.)[Abstract/Free Full Text]

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Related Article

When are randomised trials unnecessary? Picking signal from noise

Paul Glasziou, Iain Chalmers, Michael Rawlins, and Peter McCulloch
BMJ 2007 334: 349-351. [Abstract] [Full Text] [PDF]