Weighted comparison of means
BMJ 1998; 316 doi: https://doi.org/10.1136/bmj.316.7125.129 (Published 10 January 1998) Cite this as: BMJ 1998;316:129
- J Martin Bland, professor of medical statisticsa,
- Sally M Kerry, lecturer in medical statisticsb
- a Department of Public Health Sciences, St George's Hospital Medical School, London SW17 0RE
- b Division of General Practice and Primary Care, St George's Hospital Medical School, London SW17 0RE
- 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
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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 …