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| From | Ricardo Basurto-Dávila <ricardobasurto@gmail.com> |
| To | <statalist@hsphsun2.harvard.edu> |
| Subject | st: Stata 11 - Factor variables in a regression command |
| Date | Sat, 01 May 2010 01:27:44 -0400 |
I am having trouble understanding the difference between a regression that
uses a cross operator (#) and one that uses a cross factorial operator (##).
For example, this is the output I get from running two different
regressions:
. logistic y a#b
Logistic regression Number of obs = 19670
LR chi2(3) = 7.71
Prob > chi2 = 0.0525
Log likelihood = -1473.1898 Pseudo R2 = 0.0026
----------------------------------------------------------------------------
y | Odds Ratio Std. Err. z P>|z| [95% Conf. Int.]
-----------+----------------------------------------------------------------
a#b |
0 1 | 1.567419 .2804138 2.51 0.012 1.1038 2.2256
1 0 | 1.447424 .2588797 2.07 0.039 1.0194 2.0551
1 1 | 1.211988 .2246236 1.04 0.300 .84283 1.7428
----------------------------------------------------------------------------
. logistic y a##b
Logistic regression Number of obs = 19670
LR chi2(3) = 7.71
Prob > chi2 = 0.0525
Log likelihood = -1473.1898 Pseudo R2 = 0.0026
----------------------------------------------------------------------------
y | Odds Ratio Std. Err. z P>|z| [95% Conf. Int.]
-----------+----------------------------------------------------------------
1.a | 1.447424 .2588797 2.07 0.039 1.0194 2.0551
1.b | 1.567419 .2804138 2.51 0.012 1.1038 2.2256
|
a#b |
1 1 | .5342167 .1302597 -2.57 0.010 .33125 .86152
----------------------------------------------------------------------------