[EpiData-list] Asymmetric limbs of Confidence Interval - clarification

epidata-list at lists.umanitoba.ca epidata-list at lists.umanitoba.ca
Tue Feb 15 23:43:13 CST 2011

Please see below the table generated in Epidata analysis and note the
confidence interval in the first row

HIV results     Total    1539    108      7.0      (5.8-8.4)

This is the proportion of HIV infected among a group of TB suspects which is
7% with 95% CI being 5.8 - 8.4.

It can be deduced that the upper limb of confidence interval(which is 1.96
times Standard error) is 8.4 minus 7 = 1.4

Similarly, it can be deduced that the lower limb of confidence
interval(which is 1.96 times Standard error) is 7 minus 5.8 = 1.2

*Query - why are the two limbs of the confidence interval different?* the
same observation can be noted in all CIs noted above in the table. The
asymmetry of limbs of CI are expected in case of non-linear measures like
Odds Ratio, Relative risk, Hazard Ratio etc, but not in case of simple
proportions. I am given to understand (from Dr Hans Reider, from whom I
learnt Epidata) that EpiData Analysis is purported to follow the
recommendations by the BMJ  in the calculation of most confidence intervals
and this would indeed suggest that the CI around a proportion should be
symmetric on a linear scale.

Would be grateful if you could clarify.

Dr. M. V. Ajay Kumar
WHO National Consultant - RNTCP
Central TB Division, MOHFW
523'C' Wing, Nirman Bhavan, Maulana Azad Road, New Delhi - 110108
Mail - No. 353-C, J&K Pocket, Dilshad Garden, Delhi - 110095
Phone : +91-9311384574
Email : Kumaramv at rntcp.org;

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