st: Adding variables containing empty values

[Ziad El-Khatib <Ziad.El-Khatib@ki.se>]

Dear STATA lister,
Any tips how to add (summation) 3 variables when one has empty value
(example below)
I did not want to use replace var=0 if var==. because i want to know
when all the 3 variables (ndrug1,ndrug2 and ndrug3 have empty values
at the same time)


id      visitn      ndrug1      ndrug2      ndrug3      total
-----------------------------------------------------------------------------
1      2              .                2             1             3
1      4              2                .             3             5
1      8              2                3            3             8
1      12            .                 .            .              .
2      2              .                 .            2             2
2      4              1                1            .              2

etc..




Any tips?
Thank you in advance and best regards
ziad

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