This may also help: in SPSS, you can look at the Variable View of your data and indicate a missing value. ("Missing Value" is one of the columns in that view.) if you double-click on the missing value cell for a variable, it will bring up a window that allows you to specify the missing value. So in this case, if you said it was 99, then SPSS will skip any "99" entry in any analyses you do using that variable. And you can do that for as many of your variables as you like.