st: Looping a poisson regression analysis for levelsof a variable

[Tim Evans <Tim.Evans@wmciu.nhs.uk>]

Dear all,

I am running a Poisson regression on counts of surgery between two periods (Stata 11.2), however what I have is more than two periods in my data. Want I want to do is work out a loop that keeps the base period, but then cycles through the other periods. In this I have realized that my variable needs to be coded 0,1 or 1,2 etc, but my periods are 0,1,2,3,4. I cant 'keep' periods 0 and 2 as this returns incorrect IRRs.

My data are set up as follows


age     period  st      pyears
1       0       1266    1002323
2       0       3230    1687257
3       0       1251    614507
1       1       1553    1061158
2       1       4072    1665670
3       1       1517    658308
1       2       1536    1033397
2       2       4648    1739356
3       2       1700    671664
1       3       1562    1065275
2       3       5452    1799757
3       3       1770    649461
1       4       1901    1189778
2       4       6311    1900265
3       4       1765    651825

The code I have tried and failed with is this (it fails to restore the data so that I can keep period 0 and 2, period 0 and 3 and so on) At the finish I would like to age for each age group, IRR for period 1 v 0, period 2 v 0, period 3 v 0 and period 4 v 0:

levelsof period, local(levels)
levelsof age, local(age)
preserve
foreach l of local levels {
foreach a of local age {
keep if period==0 | period==`l'
tab period
replace period = 1 if period==`l'
cap nois: poisson st period if age==`a' & inlist(period, 0,`l'), exposure(pyears) irr
}
}
restore



Best wishes

Tim

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