FLAG_NANS_OCCURED checks whether the last call(s) to sumskipnan or covm contained any not-a-numbers in the input argument. A call to FLAG_NANS_OCCURED() resets also the flag whether NaN's occured. Only sumskipnan or covm can set the flag again. Because many other functions like mean, std, etc. are also using sumskipnan, also these functions can be checked for NaN's in the input data. see also: SUMSKIPNAN, COVM