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From |
"Sarah A. Mustillo" <smustillo@psych.duhs.duke.edu> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
st: Why does Stata drop an additional category? |

Date |
Tue, 17 Feb 2004 09:46:20 -0500 |

I think I am going to have to disagree with Rich on this one and say that perhaps the additional category is being dropped because of the interaction term.

Yaojun is specifying that the highest education category be used as the reference category, instead of the default, yet the lowest is being dropped anyway. Looking at his command, he is including education as a main effect, and then what appears to be a centered education variable in the interaction effect. using xi to create the interaction effect automatically includes the main effects as well though. So, I think he is entering education as a main effect twice, which is probably why that extra category is being dropped due to collinearity.

Sarah

xi: logistic unemployed i.religion i.educ5 i.cath*e5c

I would like to ask you for advice. I am doing a logit on unemployment and my independent variables are: religion 1=Protestant; 2=Catholic; 3=None cath 1=Catholic; 0=Other educ5 1=Degree; 2=Prof below degree; 3=A Levels; 4=O Levels; 5=None e5c -2=None; -1=O Level; 0=A Levels; 1=Prof below degree; 2=Degree gen cath=religion==2 if religion<=3 char religion[omit] 1 /* Protestants =base*/ char educ5[omit] 5 /* None/Prim =base*/ I wish to use the main effects of religion and educ5, and the interaction effects for cath*e5c. The first four of the following commands all end in _Ieduc5_1 dropped due to collinearity; the last gives the coefficient for this category but the model statistics seems strange: no constant and no comparability with the output in SPSS for the same data (the syntax and the results are attached). xi: logistic unemployed i.religion i.educ5 i.cath*e5c xi: logistic unemployed i.religion i.educ5 i.cath*e5c,coef xi: logit unemployed i.religion i.educ5 i.cath*e5c, or xi: logit unemployed i.religion i.educ5 i.cath*e5c xi: logit unemployed i.religion i.educ5 i.cath*e5c,nocons or I would be most grateful if anyone could explain to me why in the first four commands the first category of education (_Ieduc5_1) is dropped (I used both Stata 7 and Stata 8); and how to obtain an output similar to that in SPSS. The results for models 2-4 are not presented because they are very similar to results from model 1. Many thanks. Yours sincerely Yaojun Li ************************************************************************ **** *Stata outputs for model 1: . xi: logistic unemployed i.religion i.educ5 i.cath*e5c /*_Ieduc5_1 dropped*/ i.religion _Ireligion_1-3 (naturally coded; _Ireligion_1 omitted) i.educ5 _Ieduc5_1-5 (naturally coded; _Ieduc5_5 omitted) i.cath _Icath_0-1 (naturally coded; _Icath_0 omitted) i.cath*e5c _IcatXe5c_# (coded as above) note: _Ieduc5_1 dropped due to collinearity note: _Icath_1 dropped due to collinearity Logistic regression Number of obs = 1936 LR chi2(7) = 186.93 Prob > chi2 = 0.0000 Log likelihood = -934.26692 Pseudo R2 = 0.0909 ------------------------------------------------------------------------ ------ unemployed | Odds Ratio Std. Err. z P>|z| [95% Conf. Interval] -------------+---------------------------------------------------------- ------ _Ireligion_2 | 3.150993 .5743082 6.30 0.000 2.204475 4.503909 _Ireligion_3 | 2.095747 .6119331 2.53 0.011 1.182492 3.714322 _Ieduc5_2 | 2.469711 1.182263 1.89 0.059 .9664331 6.311325 _Ieduc5_3 | 2.554673 .851014 2.82 0.005 1.329789 4.907813 _Ieduc5_4 | .8599691 .2137221 -0.61 0.544 .5283719 1.399671 e5c | .4881296 .0769878 -4.55 0.000 .3583303 .6649465 _IcatXe5c_1 | 1.079315 .1140581 0.72 0.470 .8773971 1.3277 ------------------------------------------------------------------------ ------ (results from models 2-4 omitted) *Stata output for model 5: . xi: logit unemployed i.religion i.educ5 i.cath*e5c,nocons or i.religion _Ireligion_1-3 (naturally coded; _Ireligion_1 omitted) i.educ5 _Ieduc5_1-5 (naturally coded; _Ieduc5_5 omitted) i.cath _Icath_0-1 (naturally coded; _Icath_0 omitted) i.cath*e5c _IcatXe5c_# (coded as above) note: _Icath_1 dropped due to collinearity Iteration 0: log likelihood = -1341.9329 Iteration 1: log likelihood = -956.23273 Iteration 2: log likelihood = -936.45777 Iteration 3: log likelihood = -934.40179 Iteration 4: log likelihood = -934.26857 Iteration 5: log likelihood = -934.26692 Logit estimates Number of obs = 1936 LR chi2(8) = . Log likelihood = -934.26692 Prob > chi2 = . ------------------------------------------------------------------------ ------ unemployed | Odds Ratio Std. Err. z P>|z| [95% Conf. Interval] -------------+---------------------------------------------------------- ------ _Ireligion_2 | 3.150993 .5742976 6.30 0.000 2.204489 4.503879 _Ireligion_3 | 2.095747 .6119294 2.53 0.011 1.182496 3.714309 _Ieduc5_1 | .0037776 .0023571 -8.94 0.000 .001112 .0128333 _Ieduc5_2 | .0376322 .0088729 -13.91 0.000 .0237062 .0597387 _Ieduc5_3 | .157016 .0276982 -10.50 0.000 .1111192 .2218702 _Ieduc5_4 | .2132 .0434898 -7.58 0.000 .14294 .3179952 e5c | 1.968932 .1016632 13.12 0.000 1.779427 2.17862 _IcatXe5c_1 | 1.079315 .1140558 0.72 0.470 .8774008 1.327695 ------------------------------------------------------------------------ ---- * * For searches and help try: * http://www.stata.com/support/faqs/res/findit.html * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

Sarah A. Mustillo, Ph.D Department of Psychiatry and Behavioral Sciences Duke University School of Medicine Box 3454 Durham NC 27710 919 687-4686 x231 * * For searches and help try: * http://www.stata.com/support/faqs/res/findit.html * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**Follow-Ups**:**Re: st: Why does Stata drop an additional category?***From:*Richard Williams <Richard.A.Williams.5@nd.edu>

**References**:**st: [Why does Stata drop an additional category?]***From:*Marcello Pagano <pagano@hsph.harvard.edu>

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