Reducing number of postcodes that cannot be ascribed would increase validity of method

doi:10.1136/bmj.311.7009.876

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Ronan A Lyons,
Stephen Monaghan,
Martin Heaven,
Alan Willson

Consultant in public health medicine Senior registrar in public health medicine Information scientist in public health medicine Pharmacy adviser West Glamorgan Family Health Services Authority, Swansea SA1 1LT

EDITOR,--Glen Scrivener and David C E F Lloyd conclude that allocating census data to general practice populations is not sufficiently accurate for the purposes of explaining variations in prescribing despite their finding correlation coefficients of up to 0.84 from two of their methods.1 They acknowledge that perfect correlation is impossible in an ecological comparison. Nevertheless, the correlation coefficients might have been higher if the 11710 patients (3.2%) whose postcode could not be matched had been included in the analysis. This …