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From |
Marcello Pagano <pagano@hsph.harvard.edu> |

To |
statalist@hsphsun2.harvard.edu |

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

Date |
Tue, 17 Feb 2004 08:26:24 -0500 |

I am posting this for Yaojun. I have left off the SPSS output which is a Word file. Anyone want it, please contact Yajun (y.li.3@bham.ac.uk) or me, directly.

m.p.

Dear all,

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

------------------------------------------------------------------------

----

*

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**Follow-Ups**:**st: Why does Stata drop an additional category?***From:*"Sarah A. Mustillo" <smustillo@psych.duhs.duke.edu>

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

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

**Re: st: [Why does Stata drop an additional category?]***From:*chris wallace <c.wallace@qmul.ac.uk>

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