The trimmean:0.1 operation drops the lowest and highest 10 percent of values (5 percent off each end) before averaging, which dampens the effect of outliers. The number after the colon is the total fraction trimmed, so trimmean:0.2 removes 10 percent off each side. Trimmed means are a standard robust-statistics alternative to the plain mean. Note that the fraction comes before the field number, unlike other operations.
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