Rand Wilcox in Fundamentals Of Statistical Methods, 1st. edition, gives a formula which says that for a 20% trimmed mean, you would trim away 20% of one end of the ranked data, and 20% of the other end, making 40% trimmed away in total.
But spreadsheets such as the Calc of LibreOffice5, would for a 20% trimmed mean only trim away 10% from one end and another 10% from the other end, making 20% trimmed away in total.
Which one is right?
The author also writes that a 20% trimmed mean is best for mixture distributions. Is this correct?