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Re: st: observations differ with lrtest because of interaction terms

From   Richard Williams <>
Subject   Re: st: observations differ with lrtest because of interaction terms
Date   Tue, 18 May 2010 17:38:47 -0500

At 04:13 PM 5/18/2010, Meredith Carlson wrote:
Hello Stata users,

I'm a Stata rookie, so please bear with me... I'm having problems when I run lrtest with logistic regression to test for interaction between wealth index and region variables. The number of observations differ, not because of missing values, but because of the interaction terms. The model including interaction has fewer observations because some have been dropped because of this note...

note: _IsesXreg_3_9 != 0 predicts failure perfectly
      _IsesXreg_3_9 dropped and 4 obs not used

What exactly does this note mean? What can I do to correct the number of observations so lrtest will run?

This occurs when, for example, every time x equals 0 y also equals 0. You may, of course, want to reconsider your model, as you may be spreading the data too thin. Sometimes -exlogistic- can address the problem, at least if the sample isn't too big. There are some routines out there that offer different approaches but off the top of my head I can't remember what they are.

Richard Williams, Notre Dame Dept of Sociology
OFFICE: (574)631-6668, (574)631-6463
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