Hello Anirban,
You can think of the "fixed effects" cross-sectional time series models
like pooled (or "stacked") cross-sections, with a dummy variable for each
unit of analysis. This makes them quite different than the more
traditional kind of "fixed effect" model, which is comparing treatment
effects.
Your model probably has a different age at each time point for each
person. That is why it varies, and why it has an effect.
- Paul Millar
Sociology
University of Calgary
At 01:32 PM 02/03/2005, you wrote:
Hi,
I am running Stata 8 SE.
Does anyone know why xtnbreg, fe generates estimates for covariates that
do not vary within group. For example, my outcome variable is y, which is
a count variable and I have two period of data for every caseid.
. xtnbreg y age time, fe nolog i(caseid)
note: you are responsible for interpretation of non-count dep. variable
note: 6096 groups (12192 obs) dropped due to all zero outcomes
Conditional FE negative binomial regression Number of obs = 992
Group variable (i): caseid Number of groups = 496
Obs per group: min
= 2
avg
= 2.0
max
= 2
Wald
chi2(2) = 188.18
Log likelihood = -402.55541 Prob > chi2 = 0.0000
------------------------------------------------------------------------------
y | Coef. Std. Err. z P>|z| [95% Conf.
Interval]
-------------+----------------------------------------------------------------
age
| .0135565 .0216634 0.63 0.531 -.028903 .0560161
time
| -1.505405 .1098068 -13.71 0.000 -1.720623 -1.290188
_cons
| -2.399964 .5905975 -4.06 0.000 -3.557513 -1.242414
------------------------------------------------------------------------------
. sort caseid time
. count if caseid ==caseid[_n-1] & age !=age[_n-1]
0
Covariate age does not vary by period within caseid. What is the
interpretation of the coefficient on age? Shouldn't age drop out in a
fixed effects model?
Thanks,
Anirban
_______________________________________
Anirban Basu Ph.D.
Section of General Internal Medicine
Department of Medicine
University of Chicago
5841 S. Maryland Ave, MC-2007
Chicago IL 60637
Tel: +1 773 834 1796
Fax: +1 773 834 2238
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