Use the Five Percent Trimmed Mean to give a more representative description of data

The Five Percent Trimmed Mean, a descriptive statistic, can help law departments describe such data as their spending patterns, amounts of invoices, cycle times, and internal hours worked. The technique came to my attention in an article by Stephen J. Lubben, “Choosing Corporate Bankruptcy Counsel,” ABI Law Rev., Vol. 14, 391, at 397 (See my posts of June 30, 2006 for more on statistics of dispersion; and Nov. 30, 2005 for definitions of terms that include average, median and mean.).

When a law department has a large set of numbers, but some are very high or very low, the analyst may choose to drop the top and bottom five percent of the numbers and then average the remaining numbers. That trimming leaves the remaining numbers as more representative, less skewed by the omitted extremes. Hence, when you use the Five Percent Trimmed Mean you concentrate on the 90 percent of the data that is in the most normal range.

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