see MISSING VALUES.
It sounds like you are starting with test-strings as responses,
and that is not convenient for later analyses.
You need to use RECODE or AUTORECODE to create a numeric
variable, and put all the NA responses into one or several categories,
and declare those categories as MISSING. Then FREQ will show
the percentages in both ways, % of total and % of Valid.
If you use AUTORECODE, it gives you values in alphabetical
order, which is likely to be inconvenient, too, so you would
want a further step of creating a variable that puts the good
responses as properly ordered to 1-5, or whatever.
--
Rich Ulrich