Re: st: Why do some observations fail to be stored when I use a loopwith

[n j cox <n.j.cox@durham.ac.uk>]

I confirm the kind of behaviour you report in example 2
within Stata 8.2. So, I suspect it is nothing to
do with the version of Stata. You have been bitten, I guess,
by a limitation of -genhwi-. Why should it produce
different results with the same data? Seeing a -sort-
in the code, this suggests to me some dependence somewhere on exact sort 
order, as with ties -sort- is not guaranteed to produce
the same sort order w.r.t. the original observation
numbers. I don't know why this should make a difference,
but I can't see anything else that could differ between runs.

What is more, the proportion of results I get back
as non-missing is close to 1 - exp(-1), which may
ring bells from a probability course. -sort, stable-
seems to improve the problem, but not to eliminate it.

I'd get in touch with the author, Mario A. Cleves. He
has not been a visible member of Statalist recently.

Nick
n.j.cox@durham.ac.uk

tiago.pereira@incor.usp.br

I have found the a problem, which, not surprisingly, I have no idea why it
happens. I am using Stata 9.2 SE (Windows XP emulated in Fedora).

Concisely, I wish to run a loop, compute a statistic, and store the
results in my current dataset. In this respect, the problem relies on the
fact that some observations of the statistic of interest are not
computed/stored.


For example, suppose I want to compute a P value from an exact test. Let�s
consider the exact p value obtained by -genwhi-, a program used in
Genetics.

----- Example 1 ---------------
clear
local example 10
set obs `example'
gene a = round(uniform()*25)
gene b = round(uniform()*25)
gene c = round(uniform()*25)

qui gene p_example=.

forvalues i = 1/`example' {
     qui genhwi `=a[`i']' `=b[`i']' `=c[`i']'
     qui replace p_example = r(p_exact) in `i'
     }
---- End Example 1 -----------

Using the example just described above, I successfully get all results
without problems. Nevertheless, if I increase the number of observations,
say, local example 1000, then several observations are not
computed/stored. For example:

a	b	c	p_example
15	7	4	.5175038
7	11	3	.
23	20	18	.
8	10	22	.
0	20	20	.0000885
17	23	1	.0132223
10	12	15	1
.
.
.
6	2	20
16	16	12	.0000974
4	17	16
7	2	8	.4895405

Interestingly enough, this also happens when all observations are equal,
that is:

----- Example 2 ---------------
clear
local example 1000
set obs `example'
gene a = 25
gene b = 50
gene c = 25

qui gene p_example=.

forvalues i = 1/`example' {
     qui genhwi `=a[`i']' `=b[`i']' `=c[`i']'
     qui replace p_example = r(p_exact) in `i'
     }
---- End Example 2 -----------

which gives:


a	b	c	p_example
25	50	25	.
25	50	25	1
25	50	25	.
25	50	25	1
25	50	25	1
25	50	25	1
25	50	25	.
25	50	25	1
.
.
.
25	50	25	.
25	50	25	1
25	50	25	.


I do not have Stata 10 in my home PC, but will try to investigate if the
same problem occurs in that version of Stata. If you have any tip and/or
suggestions, I will be, as usual, appreciating your time and help.

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