Asymmetric confidence intervals.

Dear Professor Mean, I found a journal article with a confidence interval that was asymmetric. For example, the authors reported a mortality difference of 5% and a 95% confidence interval of -1.2% to 12%. I can't understand how the CI can be unequally distributed if it uses the form ESTIMATE +/- 1.96*STANDARD ERROR.

Not every confidence interval uses a plus or minus type formula. The ones that don't are usually asymmetric. In particular, most confidence intervals involving ratios are asymemmetric (though some of them are symmetric on a log scale). Also confidence intervals that take the small sample sizes into account are often asymmetric. It's good that you remember your confidence interval formula from your Statistics class, but your instructor probably didn't have time to show you every possible confidence interval formula.

This webpage was written by Steve Simon on 1999-09-03, edited by Steve Simon, and was last modified on 2008-07-08. Send feedback to ssimon at cmh dot edu or click on the email link at the top of the page. Category: Ask Professor Mean, Category: Confidence intervals