On Fri, 18 Nov 2011, Annaert Jan wrote: Yes, the concept of a "missing value" (gretl's "NA") does not apply to matrices. Matrices may contain infinities and NaNs (not-a-number) under some conditions, but they can't contain NAs. That's because we use straight IEEE arithmetic in matrix computations, and while IEEE knows how to handle infinities and NaNs it has no notion of NAs. Good point. When a matrix is fed to ok() it works as a detector of non-finite values. I'll put that information into the function reference. Note that if you really need it you can construct a function to turn non-finite elements of matrix into zeros: <hansl> function void matrix_misszero (matrix *m) loop j=1..cols(m) -q loop i=1..rows(m) -q if !ok(m[i,j]) m[i,j] = 0 endif endloop endloop end function matrix m = 3*I(2) m[2,2] = 0/0 print m matrix_misszero(&m) print m </hansl> Allin Cottrell