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st: lincom command


From   Ari Samaranayaka <ari.samaranayaka@ipru.otago.ac.nz>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: lincom command
Date   Tue, 12 Mar 2013 17:36:18 +1300

Hi folks
I need to use a poisson multivatiate model using a human health dataset, outcome is a specific health outcome, one of the explanatory variables is ethnicity. One of the results I need to estimate is IRR for various ethnicities relative to a reference ethnicity. Let us say ethnicities are A, B, C, D. Some people belong to multiple ethnicities, therefore ethnicity is not a variable with mutually exclusive categories. For this reason I cannot represent it using dummy variables. Does any one know how to represent a categorical variable when categories are not mutually exclusive?

What I have done is, have created a set of mutually exclusive ethnic categories so that i can use them in the model. Say those categories are P, Q, R, S, T (here I have more groups than above, I have no research interest on them). People from single ethnicity in original ethnicity classification now belongs to multiple categories in new classification, but new classification can be represented by dummy variables. Then I can have estimates (regression coefficients and IRR) for each of these new ethnic categories, but what I really need is the estimates for my original ethnic categories. Does any one know how to convert estimates for P, Q, R, S, T into estimates for A, B, C, D?

I thought I can use stata lincom command for that as stata documentation says. For example, required linear combination for ethnicity A is determined by the distribution of ethnic A people across ethnic groups P to T. I know those distributions for all interested ethnicities. Do you think this is a correct approach?

Thank you in advance for any help.
Regards
Ari



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