How to Use Excel's TRIMMEAN Function

Excel's TRIMMEAN function calculates the mean from a data set where a percentage of the top and bottom data points are excluded and returns this value. It is used to exclude outlying data from the analysis.

Instructions

Difficulty: Moderate

1. Learn the syntax for TRIMMEAN. It is TRIMMEAN(test_array,exclusion_percentage).
2. Provide test_array as an array of values for which to trim and calculate the mean.
3. Include exclusion_percentage as the decimal fraction of values to be excluded from the calculation. Half of this fraction will be trimmed from the top, and half will be trimmed from the bottom.
4. Examine the way exclusion_percentage is evaluated. TRIMMEAN returns #NUM! if exclusion_percentage is less than 0 or greater than 1. The total number of values to be excluded is rounded down to the nearest even number so that an equal number of values are trimmed from the top and bottom. This symmetry is maintained in order to avoid a bias.
5. Consider a set of 30 data points with an exclusion percentage of 0.1. We would have exactly 3 values. However, this number would be rounded down to 2 with one value each being excluded from the top and bottom.
6. Look at some examples of how TRIMMEAN works. If we have A2:A12 = 4,5,6,7,2,3,4,5,1,2,3 then TRIMMEAN(A2:A12,0.2) = 3.777778. Note that 11 x .2 = 2.2 so there are two points, 1 and 7 to be excluded. The remaining values are averaged.

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