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st: egen & sum()


From   Neil Shephard <nshephard@nhs.net>
To   statalist@hsphsun2.harvard.edu
Subject   st: egen & sum()
Date   Wed, 26 Nov 2008 09:51:56 +0000

Hi,

I've been poring over someone else's Stata code trying to understand it
and have discovered what seems to be inconsistent or undocumented behaviour.

The do-file has the following line...

egen b = sum(a)

This stood out to me as I thought the current version of -egen- uses the
-total()- function to obtain the combined (as opposed to running) sum of
a variable so checked the -man egen- and -man egenmore- pages and sure
enough there is no mention of -sum() as an -egen- function.

-sum()- is however a [P] function and returns the running sum of the
specified variable.

Thus I would have expected -egen b = sum(a)- to return the running sum
of b, but this is not the case, it behaves as though -sum()- is a
synonym for -total()- as the following example demonstrates...

. clear

. set obs 10
obs was 0, now 10

. gen a = _n

. gen b = sum(a)

. egen b2 = sum(a)

. egen b3 = total(a)

. list

     +-------------------+
     |  a    b   b2   b3 |
     |-------------------|
  1. |  1    1   55   55 |
  2. |  2    3   55   55 |
  3. |  3    6   55   55 |
  4. |  4   10   55   55 |
  5. |  5   15   55   55 |
     |-------------------|
  6. |  6   21   55   55 |
  7. |  7   28   55   55 |
  8. |  8   36   55   55 |
  9. |  9   45   55   55 |
 10. | 10   55   55   55 |
     +-------------------+

Based on the help-pages and documentation I would have expected b2 == b1
as -egen b2 = sum(a)- should be treating -sum()- as described in the
-man sum()- page.  Indeed even the third example in -man egen- shows
that -sum()- should be used with -generate- and -total()- should be used
with -egen-

Is there any historical legacy that anyone is aware of -sum()- being a
valid -egen- function that may be lingering around causing this behaviour?

Should the behaviour or the documentation be modified?

Or have I completely misunderstood things?

Neil

-- 
"We should make things as simple as possible, but not simpler" - Anon (not Albert Einstein)


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