Intended for healthcare professionals

Education And Debate

Inflation in epidemiology: “The proof and measurement of association between two things” revisited

BMJ 1996; 312 doi: (Published 29 June 1996) Cite this as: BMJ 1996;312:1659
  1. George Davey Smith, professor of clinical epidemiologya,
  2. Andrew N Phillips, reader in epidemiology and biostatisticsb
  1. a University of Bristol, Department of Social Medicine, Bristol BS8 2PR
  2. b University Department of Primary Care and Population Sciences, Royal Free Hospital School of Medicine, London NW3 2PF

    The details of methods of statistical analysis used in studies reported in the BMJ will often be skimmed rapidly by readers who want to quickly assimilate the main message. The exact nature of the statistical methods may become a focus of attention, but this can be seen as an arcane area, of interest perhaps to the specialist and pedant, but not to the general reader. Increasingly, however, the particular details of analytical methods can greatly influence the apparent nature and importance of the findings. This can be illustrated by reference to the recent paper and commentaries in the BMJ regarding new analyses of Intersalt data.

    One potentially contentious area is the manner in which the association between sodium excretion and blood pressure has been “corrected for regression dilution bias.” For many readers the basic principle of dealing with the underestimation of associations caused by poor measurement may seem reasonable, but the validity of applying the particular corrections which are used has to be taken on trust. Confusion may be increased by the presentation of a set of “updated” corrected estimates, to replace the already corrected estimates given in the initial Intersalt report.1

    Comparison of the results reported in different ways reveals the degree to which such “corrections” can alter the picture. The difference in systolic blood pressure associated with 100 mmol higher 24 hour urinary sodium excretion is presented. The latter represents a considerable difference in sodium excretion—roughly two standard deviations in the British Intersalt centres or the difference between the means in the Kenyan and the British centres. In the original report the estimated blood pressure difference across this range was 1.6 mm Hg, which was reduced to 1.0 mm Hg on adjustment for body mass index, alcohol intake, and urinary potassium excretion. Applying an adjustment for regression …

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