How to Build a Small-Sample Confidence Interval with CONFIDENCE.T in Excel

Use CONFIDENCE.T with STDEV.S and COUNT to put an honest margin of error on a small pilot study.

You will learn to

• Calculate a t-based margin of error straight from raw data
• Find the lower confidence limit
• Compare the t margin with the normal margin

How it works

CONFIDENCE.T returns the margin of error for an average using the t distribution, which is the right choice when the sample is small and the standard deviation is estimated from that same sample. Feed it alpha, STDEV.S of the data and COUNT of the data.

The result is always a little wider than CONFIDENCE.NORM for the same inputs, because the t distribution allows for the extra uncertainty in a small sample. The gap shrinks as the sample grows; with 9 data points it is about 18% wider.

=CONFIDENCE.T(0.05,STDEV.S(B2:B10),COUNT(B2:B10))