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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