trimmed-mean (data percentage &key start end key)
Returns a trimmed mean of data.&#39; A trimmed mean is an ordinary, arithmetic<br> mean of the data, except that an outlying percentage has been discarded. For<br> example, suppose there are ten elements in data,' and percentage&#39; is 0.1: the<br> result would be the mean of the middle eight elements, having discarded the<br> biggest and smallest elements. If percentage' doesn't result in a whole number
of elements being discarded, then a fraction of the remaining biggest and
smallest is discarded. For example, suppose data&#39; is &#39;(1 2 3 4 5) and<br> percentage' is 0.25: the result is (.75(2) + 3 + .75(4))/(.75+1+.75) or 3. By
convention, the 0.5 trimmed mean is the median, which is always returned as a
number.