These periods usually indicate system missing values, see http://www.spss-tutorials.com/spss-miss ... al/#sysmis
Usually, zeroes and ones are simply represented by (and shown as) zeroes and ones. Although it's unusual, it could
be that in your case zeroes are represented by system missing values. If you really are very sure that all
system missing values represent zeroes, you can RECODE them into zeroes like
RECODE v1 v2 v3 (sysmis = 0).
If you do so, it may be wise to RECODE into new variables, see (see http://www.spss-tutorials.com/spss-recode-command/
). Or, better, clone the original variables (http://www.spss-tutorials.com/clone-variables/
) and then RECODE.