Hi, I stumbled over the (correctly documented) behavior of pmean() with a second argument: "If the optional second argument is provided then observations for which the value of mask is zero are ignored." This means in particular that if the mask value is missing then the obs is *not* ignored because mask is not equal to zero. So this is not equivalent to the condition that only obs with 'mask != 0' should be considered, because this condition would not evaluate to True if mask is a missing value. Is this a bug or a feature, i.e. intended or not? As I said, the description matches the behavior, but to my personal taste it's a bit surprising. thanks, sven