**Asymmetric confidence intervals (created 1999-09-03)**

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*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
*

**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.