Weighted comparison of means

doi:10.1136/bmj.316.7125.129

From trainee to consultant, BMJ Group offers doctors around the world tailored information, special events, learning resources and recruitment services at every step along their career path.

... by doctors, for doctors, for patients
We are open for entries!

Online learning

BMJ Learning

High-quality CME / CPD for doctors and other healthcare professionals. BMJ Learning features hundreds of accredited, peer-reviewed learning modules in text, video, and audio formats. Find out more

Courses and Qualifications

BMJ Masterclasses

BMJ Masterclasses, led by experts, help clinicians to use the latest evidence and recent guidelines in practice and meet their CPD/CME requirements. Find out more

Exam Preparation

The leading provider of online exam preparation, helping over 167,000 healthcare professionals to pass their exams. Find out more

onExamination

Search all BMJ education articles:

From

Limit by

Access to the full text of this article requires a subscription or payment. Please log in or subscribe below.

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

View this table:

In this window
In a new window

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 …