st: Collapse with sum function and missing values

[Michael Mitchell <Michael.Norman.Mitchell@gmail.com>]

Greetings

  I am befuddled by the following example that uses the -collapse-
command with the -(sum)- function...

. clear

. input group x

          group           x
  1. 1 10
  2. 1 11
  3. 1 12
  4. 2 20
  5. 2 21
  6. 2 22
  7. 3 .
  8. 4 .
  9. 4 .
 10. end

.
. collapse (sum) x , by(group)

. list

     +------------+
     | group    x |
     |------------|
  1. |     1   33 |
  2. |     2   63 |
  3. |     3    0 |
  4. |     4    0 |
     +------------+

  Shouldn't the value of -x- for groups 3 and 4 be missing, not zero.
To me, the sum of a series of missing values is a missing value. I am
doing a collapse for about 100 variables (100 x values) and need the
value to be defined as missing (not 0) in such cases. Any ideas?

Thanks,

Michael

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