I used a collapse command to count the number of occurrences linked to particular survey responses:
collapse (count) qn1b if qn1b == 4, by (hhid)

I then saved this as a separate data file (data2) - but I'm having trouble merging it back together with my initial data set (data1) while preserving missing as well as 0 values. I'm new to merging data sets and not sure what I'm doing wrong in the merge.