Statistical significance and confidence intervals

doi:10.1136/bmj.b3401

Published 2 September 2009, doi:10.1136/bmj.b3401
Cite this as: BMJ 2009;339:b3401

Endgames

Statistical question

Statistical significance and confidence intervals

Philip M Sedgwick, senior lecturer

¹ St George’s Hospital, University of London, London SW17 0RE

p.sedgwick@sgul.ac.uk

The first 150 words of the full text of this article appear below.

A prospective population based cohort study investigated theassociation between occupation and pregnancy outcome for motherswho had missed antenatal visits. The mean birth weight was 3371g (95% confidence interval 3274 to 3469) and 3227 g (3140 to3315 g) for babies born to mothers in non-manual and manualjobs, respectively.

On the basis of the above information, which of the followingcan be deduced for mothers who missed antenatal visits?

a) In the population, 95% of mothers with a non-manual occupationhave a baby with a birth weight between 3274 g and 3469 g

b)Because the 95% confidence intervals overlap, the mean differencein birth weight between mothers in non-manual and manual jobsis not statistically significant at the 5% level

c) On average,mothers with a non-manual job had heavier babiesthan thosewith a manual job

d) If a mother has a manual job, this . . . [Full text of this article]

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On average a poor choice of words: Stephen B Lambert
bmj.com, 3 Sep 2009 [Full text]
Author's reply: Philip M Sedgwick
bmj.com, 1 Oct 2009 [Full text]