st: Stata command to compare linear combinations of predicted values after regression

[Ari Samaranayaka <ari.samaranayaka@ipru.otago.ac.nz>]

Hi folks,
Does anyone know a stata command to get incidence rate ratio (IRR) of 
one group relative to another group after poisson regression when none 
of the groups are directly represented in the model, but both groups can 
be defined by linear combinations of groups in the model?

The model is in the form (a simplified version) :
xt:poisson y i.x
where x is a variable with mutually exclusive categories.

The groups I want to compare are different linear combinations of 
categories of x. These groups cannot be directly included in the model 
because they are not mutually exclusive. For example, in the table below 
P Q R S are categories in variable X; groups I want to compare are A and 
B; numbers in the table are the number of observations.

            P     Q     R     S
Group A    25    30    35    40
Group B    50    10    15    60

Therefore:
observations in group A includes 25 from P, 30 from Q, 35 from R, 40 from S.
observations in group B includes 50 from P, 10 from Q, 15 from R, 60 
from S.
Groups A and B are not mutually exclusive.

I know lincom command can be used to get IRR for one linear combination 
of regression coefficients relative to another linear combination of 
coefficients. But none of the A or B groups are linear combinations of 
coefficients, they are linear combinations of number of observations 
corresponding to coefficients from the model. Although we can do hand 
calculations to get IRR by comparing predicted values in groups A and B, 
I am looking for a stata command for this because in actual analysis I 
have very large number of combinations to deal with.
Thanks for any help.
Ari



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