ref: list of numbers whose mean you want to calculate
fraction: fraction of the data set excluded from the mean
If fraction=0.2 and the data set contains 40 numbers, 8 numbers are trimmed from the data set (40 x 0.2): the 4 largest and the 4 smallest. To avoid a bias, the number of points to be excluded is always rounded down to the nearest even number.
This function is Excel compatible.