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Education And Debate

Weighted comparison of means

BMJ 1998; 316 doi: (Published 10 January 1998) Cite this as: BMJ 1998;316:129
  1. J Martin Bland, professor of medical statisticsa,
  2. Sally M Kerry, lecturer in medical statisticsb
  1. a Department of Public Health Sciences, St George's Hospital Medical School, London SW17 0RE
  2. b Division of General Practice and Primary Care, St George's Hospital Medical School, London SW17 0RE
  1. Correspondence to: Professor Bland

    In a recent Statistics Note1 we referred to a weighted two sample t test. Here we describe how it is done. The data were the percentage of requests from general practitioners for x ray examinations which were judged appropriate (table 1), where general practitioners had been randomised to intervention or control groups.2

    View this table:
    Table 1

    Number of requests conforming to guidelines for each practice in the intervention and control groups

    If we compare the two sets of percentages by the usual two sample t method, each observation (practice) has an equal impact on the result. As some practices contributed fewer requests than others, we wish these practices to have a lesser effect on the estimate of the difference. We can do this by weighting the practices by the …

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