Weighted comparison of means

Abstract

J Martin Bland, professor of medical statisticsa, Sally M Kerry, lecturer in medical statisticsba Department of Public Health Sciences, St George's Hospital Medical School, London SW17 0REb Division of General Practice and Primary Care, St George's Hospital Medical School, London SW17 0RECorrespondence to: Professor BlandIn 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.2View this table:In this windowIn a new windowTable 1 Number of requests conforming to guidelines for each practice in the intervention and control groupsIf 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 …