== Summing partially missing cases in SPSS ==

The default for obtaining sums and means in SPSS is to work them out only for cases with complete data.

To sum over cases, with at least x complete variable values in SPSS, we use the function, sum.x. Similarly to obtain the mean for cases, with at least x complete variable values, we use mean.x. e.g. 
{{{
compute scoresum   =  sum.3(a,b,c).
compute scoremn     =  mean.2(a,b,c).
exe.
}}}

scoresum will return missing for cases with <3 non-missing values. Ie any non-missing a, b and c. 
scoremn will return missing for cases  with <2 non-missing values, ie having two or three missing values of any of variables a or b or c.