I run a fixed effects regression in a linear probability model of self rated health and local employment change over three waves, as follows:
Code:
. xtreg binary_health_y psum_unemployed_total_cont_y calt3_other_children_y0 i.current_county_y1 i.year i.o
> wn_education_y age_y if has_y0_questionnaire==1 & has_y5_questionnaire==1 | has_y0_questionnaire==1 & has
> _y10_questionnaire==1 | has_y0_questionnaire==1 & has_y5_questionnaire==1 & has_y10_questionnaire==1 | ha
> s_y0_questionnaire==1 & cbmi_y5 !=. & has_y5_questionnaire==0 | has_y0_questionnaire==1 & cbmi_y10 !=. &
> has_y10_questionnaire==0 | has_y0_questionnaire==1 & cbmi_y5 !=. & has_y5_questionnaire==0 & cbmi_y10 !=.
> & has_y10_questionnaire==0 | has_y0_questionnaire==1 & cbmi_y5 !=. & has_y5_questionnaire==1 | has_y0_qu
> estionnaire==1 & cbmi_y10 !=. & has_y10_questionnaire==1 | has_y0_questionnaire==1 & cbmi_y5 !=. & has_y5
> _questionnaire==1 & cbmi_y10 !=. & has_y10_questionnaire==1, cluster (current_county_y1) fe robust
note: calt3_other_children_y0 omitted because of collinearity
note: 3.current_county_y1 omitted because of collinearity
note: 4.current_county_y1 omitted because of collinearity
note: 5.current_county_y1 omitted because of collinearity
note: 6.current_county_y1 omitted because of collinearity
note: 7.current_county_y1 omitted because of collinearity
note: 8.current_county_y1 omitted because of collinearity
note: 9.current_county_y1 omitted because of collinearity
note: 10.current_county_y1 omitted because of collinearity
note: 11.current_county_y1 omitted because of collinearity
note: 12.current_county_y1 omitted because of collinearity
note: 13.current_county_y1 omitted because of collinearity
note: 14.current_county_y1 omitted because of collinearity
note: 15.current_county_y1 omitted because of collinearity
note: 16.current_county_y1 omitted because of collinearity
note: 17.current_county_y1 omitted because of collinearity
note: 18.current_county_y1 omitted because of collinearity
note: 19.current_county_y1 omitted because of collinearity
note: 20.current_county_y1 omitted because of collinearity
note: 21.current_county_y1 omitted because of collinearity
note: 22.current_county_y1 omitted because of collinearity
note: 23.current_county_y1 omitted because of collinearity
note: 24.current_county_y1 omitted because of collinearity
note: 25.current_county_y1 omitted because of collinearity
note: 26.current_county_y1 omitted because of collinearity
note: 27.current_county_y1 omitted because of collinearity
note: 28.current_county_y1 omitted because of collinearity
note: 29.current_county_y1 omitted because of collinearity
note: 30.current_county_y1 omitted because of collinearity
note: 10.year omitted because of collinearity
note: 3.own_education_y omitted because of collinearity
note: 4.own_education_y omitted because of collinearity
note: 5.own_education_y omitted because of collinearity
note: 6.own_education_y omitted because of collinearity
Fixed-effects (within) regression Number of obs = 1578
Group variable: id Number of groups = 635
R-sq: within = 0.0066 Obs per group: min = 1
between = 0.0062 avg = 2.5
overall = 0.0047 max = 3
F(3,28) = 4.34
corr(u_i, Xb) = -0.0538 Prob > F = 0.0124
(Std. Err. adjusted for 29 clusters in current_county_y1)
----------------------------------------------------------------------------------------------------------
| Robust
binary_health_y | Coef. Std. Err. t P>|t| [95% Conf. Interval]
-----------------------------------------+----------------------------------------------------------------
psum_unemployed_total_cont_y | -.0108672 .0043554 -2.50 0.019 -.0197888 -.0019456
calt3_other_children_y0 | 0 (omitted)
|
current_county_y1 |
Cavan | 0 (omitted)
Clare | 0 (omitted)
Cork | 0 (omitted)
Donegal | 0 (omitted)
Dublin City | 0 (omitted)
Dún Laoghaire-Rathdown | 0 (omitted)
Fingal | 0 (omitted)
Galway | 0 (omitted)
Galway City | 0 (omitted)
Kerry | 0 (omitted)
Kildare | 0 (omitted)
Kilkenny | 0 (omitted)
Laois | 0 (omitted)
Limerick | 0 (omitted)
Longford | 0 (omitted)
Louth | 0 (omitted)
Mayo | 0 (omitted)
Meath | 0 (omitted)
Monaghan | 0 (omitted)
Offaly | 0 (omitted)
Roscommon | 0 (omitted)
Sligo | 0 (omitted)
South Dublin | 0 (omitted)
Tipperary North | 0 (omitted)
Waterford | 0 (omitted)
Westmeath | 0 (omitted)
Wexford | 0 (omitted)
Wicklow | 0 (omitted)
|
year |
5 | -.0879273 .0277405 -3.17 0.004 -.1447511 -.0311034
10 | 0 (omitted)
|
own_education_y |
Some secondary school | 0 (omitted)
Complete secondary education | 0 (omitted)
Some third level education at college.. | 0 (omitted)
Complete third level education at col.. | 0 (omitted)
|
age_y | .0074049 .0038814 1.91 0.067 -.0005457 .0153555
_cons | .6171357 .0901617 6.84 0.000 .4324479 .8018235
-----------------------------------------+----------------------------------------------------------------
sigma_u | .35499044
sigma_e | .35438184
rho | .50085794 (fraction of variance due to u_i)
----------------------------------------------------------------------------------------------------------As Stata omitted this variable automatically I figure that no harm is done, however, when I manually remove it from the regression my results are changed, as below:
Code:
. xtreg binary_health_y psum_unemployed_total_cont_y calt3_other_children_y0 i.current_county_y1 i.year age
> _y if has_y0_questionnaire==1 & has_y5_questionnaire==1 | has_y0_questionnaire==1 & has_y10_questionnaire
> ==1 | has_y0_questionnaire==1 & has_y5_questionnaire==1 & has_y10_questionnaire==1 | has_y0_questionnaire
> ==1 & cbmi_y5 !=. & has_y5_questionnaire==0 | has_y0_questionnaire==1 & cbmi_y10 !=. & has_y10_questionna
> ire==0 | has_y0_questionnaire==1 & cbmi_y5 !=. & has_y5_questionnaire==0 & cbmi_y10 !=. & has_y10_questio
> nnaire==0 | has_y0_questionnaire==1 & cbmi_y5 !=. & has_y5_questionnaire==1 | has_y0_questionnaire==1 & c
> bmi_y10 !=. & has_y10_questionnaire==1 | has_y0_questionnaire==1 & cbmi_y5 !=. & has_y5_questionnaire==1
> & cbmi_y10 !=. & has_y10_questionnaire==1, cluster (current_county_y1) fe robust
note: calt3_other_children_y0 omitted because of collinearity
note: 3.current_county_y1 omitted because of collinearity
note: 4.current_county_y1 omitted because of collinearity
note: 5.current_county_y1 omitted because of collinearity
note: 6.current_county_y1 omitted because of collinearity
note: 7.current_county_y1 omitted because of collinearity
note: 8.current_county_y1 omitted because of collinearity
note: 9.current_county_y1 omitted because of collinearity
note: 10.current_county_y1 omitted because of collinearity
note: 11.current_county_y1 omitted because of collinearity
note: 12.current_county_y1 omitted because of collinearity
note: 13.current_county_y1 omitted because of collinearity
note: 14.current_county_y1 omitted because of collinearity
note: 15.current_county_y1 omitted because of collinearity
note: 16.current_county_y1 omitted because of collinearity
note: 17.current_county_y1 omitted because of collinearity
note: 18.current_county_y1 omitted because of collinearity
note: 19.current_county_y1 omitted because of collinearity
note: 20.current_county_y1 omitted because of collinearity
note: 21.current_county_y1 omitted because of collinearity
note: 22.current_county_y1 omitted because of collinearity
note: 23.current_county_y1 omitted because of collinearity
note: 24.current_county_y1 omitted because of collinearity
note: 25.current_county_y1 omitted because of collinearity
note: 26.current_county_y1 omitted because of collinearity
note: 27.current_county_y1 omitted because of collinearity
note: 28.current_county_y1 omitted because of collinearity
note: 29.current_county_y1 omitted because of collinearity
note: 30.current_county_y1 omitted because of collinearity
note: 10.year omitted because of collinearity
Fixed-effects (within) regression Number of obs = 1590
Group variable: id Number of groups = 641
R-sq: within = 0.0063 Obs per group: min = 1
between = 0.0064 avg = 2.5
overall = 0.0049 max = 3
F(3,28) = 4.38
corr(u_i, Xb) = -0.0423 Prob > F = 0.0119
(Std. Err. adjusted for 29 clusters in current_county_y1)
----------------------------------------------------------------------------------------------
| Robust
binary_health_y | Coef. Std. Err. t P>|t| [95% Conf. Interval]
-----------------------------+----------------------------------------------------------------
psum_unemployed_total_cont_y | -.0101514 .0042467 -2.39 0.024 -.0188504 -.0014524
calt3_other_children_y0 | 0 (omitted)
|
current_county_y1 |
Cavan | 0 (omitted)
Clare | 0 (omitted)
Cork | 0 (omitted)
Donegal | 0 (omitted)
Dublin City | 0 (omitted)
Dún Laoghaire-Rathdown | 0 (omitted)
Fingal | 0 (omitted)
Galway | 0 (omitted)
Galway City | 0 (omitted)
Kerry | 0 (omitted)
Kildare | 0 (omitted)
Kilkenny | 0 (omitted)
Laois | 0 (omitted)
Limerick | 0 (omitted)
Longford | 0 (omitted)
Louth | 0 (omitted)
Mayo | 0 (omitted)
Meath | 0 (omitted)
Monaghan | 0 (omitted)
Offaly | 0 (omitted)
Roscommon | 0 (omitted)
Sligo | 0 (omitted)
South Dublin | 0 (omitted)
Tipperary North | 0 (omitted)
Waterford | 0 (omitted)
Westmeath | 0 (omitted)
Wexford | 0 (omitted)
Wicklow | 0 (omitted)
|
year |
5 | -.0840781 .0273625 -3.07 0.005 -.1401278 -.0280285
10 | 0 (omitted)
|
age_y | .0067429 .0038121 1.77 0.088 -.0010659 .0145516
_cons | .6316924 .088782 7.12 0.000 .4498308 .8135541
-----------------------------+----------------------------------------------------------------
sigma_u | .3549904
sigma_e | .35478865
rho | .50028423 (fraction of variance due to u_i)
----------------------------------------------------------------------------------------------I don't know why this is the case, I thought the fixed effects model was automatically omitting the variable, based on the first output where it said the variable was omitted?
My questions are:
- Why do my results change?
- Was this variable adding something to the analysis (even though it appeared only in Wave 1, I noticed that categories 3, 4, 5 and 6 were omitted, while categories 1 and 2 remained).
- Should I keep this variable or manually remove it from my regression, and why?
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