How to Build a Confidence Interval with CONFIDENCE.NORM in Excel

Use CONFIDENCE.NORM to get the margin of error around an average when the population standard deviation is known.

You will learn to

• Calculate a margin of error
• Turn it into a lower confidence limit
• Convert a confidence level into alpha

How it works

CONFIDENCE.NORM returns the margin of error for an average: the "±" in "7.8 minutes ± 0.78". It needs alpha (1 minus the confidence level), the standard deviation and the sample size. Subtract it from the sample average for the lower limit and add it for the upper limit.

Use CONFIDENCE.NORM when the standard deviation is well known from lots of history or the sample is large. For small samples where the SD is estimated from the sample itself, use CONFIDENCE.T, which gives a slightly wider, safer interval.

=CONFIDENCE.NORM(0.05,3.2,64)