st: sum of missing values returned as zero

[Jacob Wegelin <jwegelin@vcu.edu>]

The following code

clear 
set obs 4
gen Finished=.
list
summarize Finished
return list

returns a value of zero for the sum of Finished, as is seen below.  I do not understand. I would think that the sum of a list of numbers, at least one of which is missing, would be missing, not zero. Can someone explain?

. set obs 4
obs was 0, now 4

. gen Finished=.
(4 missing values generated)

. list

     +----------+
     | Finished |
     |----------|
  1. |        . |
  2. |        . |
  3. |        . |
  4. |        . |
     +----------+

. summarize Finished

    Variable |       Obs        Mean    Std. Dev.       Min        Max
-------------+--------------------------------------------------------
    Finished |         0

. return list

scalars:
                  r(N) =  0
              r(sum_w) =  0
                r(sum) =  0


Thanks

Jacob A. Wegelin
jwegelin@vcu.edu 
Assistant Professor
Department of Biostatistics
Virginia Commonwealth University
730 East Broad Street Room 3006
P. O. Box 980032
Richmond VA 23298-0032
U.S.A. 
http://www.people.vcu.edu/~jwegelin

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