Misuse of confidence intervals threatens conclusions

doi:10.1136/bmj.39212.685637.3A

BMJ 2007;334:1020 (19 May), doi:10.1136/bmj.39212.685637.3A

Letters

$beta$ blockers

Misuse of confidence intervals threatens conclusions

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The data presented in table 1 of Ong's review leave considerableuncertainty about whether atenolol is better or worse than other $beta$ blockers.¹ The confidence intervals for the results on other $beta$ blockers are wide (as fewer patients have been studied), andthe test for interaction shows that the relative risk for atenolol,compared with other $beta$ blockers, for stroke is 1.05 (95% confidenceinterval 0.26 to 4.17), for myocardial infarction 1.22 (0.91to 1.63), and for total mortality 1.21 (0.95 to 1.14). All ofthese confidence intervals include the possibility of no difference,and for stroke the results are compatible with atenolol beingfour times better or four times worse than other $beta$ blockers.It is very misleading to draw conclusions based on whether significanceis achieved with either treatment alone.²

Christopher J Cates, general practitioner

Manor View Practice, Bushey WD23 2NN

chris.cates@nhs.net

Competing interests: None declared.

Ong HT. $beta$ blockers in hypertension and cardiovascular disease. BMJ 2007;334:946-9. (5 May.)[Free Full Text]
Altman DG, Bland JM. Interaction revisited: the difference between two estimates. BMJ 2003;326:219.[Free Full Text]

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