Published 5 November 2008, doi:10.1136/bmj.a2295
Cite this as: BMJ 2008;337:a2295

Endgames

Statistical question

Relative risk

John Fletcher, clinical epidemiologist

1 BMJ, London WC1H 9JR

jfletcher{at}bmj.com

A trial of antibiotics for treating community acquired pneumonia showed that the relative risk of death was 0.5 with a P value of 0.01. Which one of the following is a possible 95% confidence interval for the relative risk?

a) (–0.8 to –0.3)

b) (0.2 to 1.6)

c) (0.3 to 0.8)

d) (1.2 to 1.6)

Show Answers

Answer

c—Relative risks are the result of dividing two risks greater than zero so a relative risk is never negative. Any confidence interval must contain the result within it. As the P value is small the 95% confidence interval cannot include the measure of "no effect," which for a relative risk is 1. The only answer satisfying all these conditions is c.

Cite this as: BMJ 2008;337:a2295


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