> David Winter writes (in part):
> > I have written a program to calculate a two-tailed poisson statistic,
> >
> > In a -.do- file, I run the commands
> >
> > gen PoissStat = .
> > by sex ageband: poissontt Observed Expected PoissStat
> >
> > Stata diligently loops through the data and using -set trace- I have
> > tracked the iterations. It does not appear to pick up the values of
> > Observed and Expected for each record, rather those of the first
> > record only. The calculation is correct for record 1 (Obs=5,
> > Expect=.258,
> > poisson(two-tailed=.0000153) and the PoissStat variable is updated in
> > each record, but with the value for record 1 only.
> >
> > Is there something I am missing either in the program or in using the
> > -by- command?
Yes, you failed to use mark or marksample in the program.
As the help for byable indicates:
"the sample is automatically restricted to the appropriate subsample
when myprog uses the mark or marksample commands; see help mark."
With 3 changes, the program becomes:
program poissontt, sortpreserve byable(recall, noheader)
/* make program usable with 'by' command - see manual [P] page 12 */
version 8
marksample touse /* added to allow subsetting data */
quietly
gsort - `touse' /* <- added to bring selected record to top of file so */
local poissval = 0 /* observed = `1', which is really observed = `1'[1], */
local minprob = 0 /* loads the correct O (and expected loads correct E) */
local maxprob = 0
local observed = `1' /* O */
local expected = `2' /* E */
local replacevar = "replace `3' = "
/* probability Observed no >= O */
if `observed' == 0 {
local maxprob = 1
}
else if `expected' < 0.000001 {
local maxprob = 0.000001
}
else {
local I = 0
local sumcalc = 0
local tempcalc = 0
local endloop = `observed' - 1
while `I' <= `endloop' {
local tempcalc = ((exp(-1 * `expected')) * (`expected' ^ `I')) / exp(lnfactorial(`I'))
local sumcalc = `sumcalc' + `tempcalc'
local I = `I' + 1
}
local maxprob = 1 -`sumcalc'
}
/* probability Observed no <= O */
local I = 0
local sumcalc = 0
local tempcalc = 0
local endloop = `observed'
while `I' <= `endloop' {
local tempcalc = ((exp(-1 * `expected')) * (`expected' ^`I')) / exp(lnfactorial(`I'))
local sumcalc = `sumcalc' + `tempcalc'
local I = `I' + 1
}
local minprob = `sumcalc'
/* evaluate probsbility to calculate statistic */
if `observed' == 0 & `expected' == 0 {
local poissval = 0 /* empty */
}
else {
if `minprob' <= `maxprob' {
local poissval = `minprob'
}
else {
local poissval = `maxprob'
}
local poissval = 2 * `poissval'
if `poissval' > 1 {
local poissval = 1
}
}
`replacevar' `poissval' if `touse' /* changed to replace only in current record */
noisily
end
And you get:
.. list
+-------------------------------------------------+
| sex ageband observed expected PoissS~t |
|-------------------------------------------------|
1. | MALE 5 5 .2585 .0000155 |
2. | MALE 10 11 .6054 1.15e-10 |
3. | MALE 15 6 .6516 .000122 |
4. | MALE 20 5 .6435 .0010803 |
+-------------------------------------------------+
Tom
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