## Function Trimmed-Mean

(

**trimmed-mean** &rest < args > )

trimmed-mean (data percentage &key start end key)

Returns a trimmed mean of ` data.' 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' 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' is '(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.