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st: Regress question group omitted


From   Lars Folkestad <lfolkestad@health.sdu.dk>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   st: Regress question group omitted
Date   Fri, 1 Jun 2012 17:38:15 +0200

Dear Statalist

I have a dataset containing two variables (continious) that record time in seconds to an event. I also have a continious variabe (charge time) that is group dependente and a grouping variable

First second charge group
16.      145.     8.          1
12.      157.      5.         2

And so forth

I want to compare time to first event between The two groups, but takeing into account The differences in charge time.

So first a stats question and then a Stata question

1. Would it be correct to just substract The charge time variable from The time to first event variable (both in sec) where the time to first event is meassured and The charge time is fixed by group (its 8s for group 1 and 5s for group 2). This so that The difference between groups found, are not due to The difference in charge time but other factors. Or do i need a multible linier regression model?

2) when doing a regression model using Stata 12 using

regress first chargetime i.group

In the results group is omitted why is this? 

Thank you
Lars
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