st: RE: Changing base level doesnt affect co-efficient values in other parts of logistic regression

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

To answer my own question, I guess had I introduced interaction terms between the variables, I would then have expected different values for these, once the base level was changed in one variable.


-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Tim Evans
Sent: 12 November 2012 08:54
To: statalist@hsphsun2.harvard.edu
Subject: st: Changing base level doesnt affect co-efficient values in other parts of logistic regression

Hi all,

I'm running a logistical regression model in Stata 11.2 to assess the likelihood of presenting early with a disease and have a number of independent categorical variables such as age, deprivation, region of residence and co-morbidity. Currently my default base level for the age group is the age group which has the largest number of observation (for instance, 60-69), which I understand to be a norm for Stata (and perhaps other programs). On reflection I thought that my model would make more sense if I set the base level to the specific age group of interest (for instance 50-59 using the char omit code). I was surprised that when I ran the two models, the only coefficients to change in the two models were the age group ones. I at least thought that there might be some movement in the coefficients of the other variables between the two models - why is this not the case?

Best wishes

Tim

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