Speaking Stata: Trimming to taste
Nicholas J. Cox
Department of Geography
Durham University
Durham, UK
[email protected]
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Abstract. Trimmed means are means calculated after setting aside zero or more
values in each tail of a sample distribution. Here we focus on trimming equal numbers
in each tail. Such trimmed means define a family or function with mean and
median as extreme members and are attractive as simple and easily understood
summaries of the general level (location, central tendency) of a variable. This
article provides a tutorial review of trimmed means, emphasizing the scope for
trimming to varying degrees in describing and exploring data. Detailed remarks
are included on the idea’s history, plotting of results, and confidence interval procedures.
Examples are given using astronomical and medical data. The new Stata
commands trimmean and trimplot are also included.
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Nicholas J. Cox
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trimmean, trimplot, trimming, means, medians, midmeans, Winsorizing, robust, resistant, graphics
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