I am running the following regression with an interaction between a categorical education variable and continuous variable. The categorical variable takes on the values (1 = no education, 2 = primary education, 3 = secondary education, 4 = university education). When I include imports non-interacted as a control, the categorical variable omits the base group (Output1).
However, when I use total trade as the control instead, it keeps the base group (Output 2). How can I understand this, and is it possible to interpret the coefficients on the interaction in Output 2?
Thank you in advance.
Ray
Code:
reg lrhourlywage schoolinglevel1#c.ltotalexports_china ltotaltrade_china ltotaltrade_eu15 ltotaltrade_usa ltotaltrade_mca ltotaltrade_row i.schoolinglevel1 i.region i.occupationgroup i.isic1 i.establishmentsize _2015 _2016 _2017 _2018 [fweight = factor] , vce(cluster region) , vce(cluster region)
Code:
Linear regression Number of obs = 1,517,202
F(4, 5) = .
Prob > F = .
R-squared = 0.4382
Root MSE = .4583
(Std. Err. adjusted for 6 clusters in region)
-------------------------------------------------------------------------------------------------------
| Robust
lrhourlywage | Coef. Std. Err. t P>|t| [95% Conf. Interval]
--------------------------------------+----------------------------------------------------------------
schoolinglevel1#c.ltotalexports_china |
2 | .0055916 .0055252 1.01 0.358 -.0086114 .0197947
3 | .0146709 .0162778 0.90 0.409 -.0271724 .0565142
4 | .0720004 .0125011 5.76 0.002 .0398653 .1041356
|
ltotalexports_china | -.027375 .012245 -2.24 0.076 -.0588519 .0041019
ltotalimports_china | .0288434 .024478 1.18 0.292 -.0340792 .091766
ltotaltrade_eu15 | -.0073836 .0710008 -0.10 0.921 -.1898971 .1751298
ltotaltrade_usa | .0914549 .0475511 1.92 0.112 -.0307791 .213689
ltotaltrade_mca | .038949 .0462096 0.84 0.438 -.0798365 .1577346
ltotaltrade_row | .0495398 .0261668 1.89 0.117 -.0177242 .1168038Output 2
Code:
Linear regression Number of obs = 1,517,202
F(4, 5) = .
Prob > F = .
R-squared = 0.4383
Root MSE = .45823
(Std. Err. adjusted for 6 clusters in region)
-------------------------------------------------------------------------------------------------------
| Robust
lrhourlywage | Coef. Std. Err. t P>|t| [95% Conf. Interval]
--------------------------------------+----------------------------------------------------------------
schoolinglevel1#c.ltotalexports_china |
1 | -.0315226 .0112729 -2.80 0.038 -.0605005 -.0025447
2 | -.0256822 .0071686 -3.58 0.016 -.0441096 -.0072549
3 | -.0167331 .0064504 -2.59 0.049 -.0333145 -.0001517
4 | .0404786 .0129226 3.13 0.026 .00726 .0736971
|
ltotaltrade_china | .1049092 .0460189 2.28 0.072 -.0133862 .2232047
ltotaltrade_eu15 | .0343422 .0780796 0.44 0.678 -.1663679 .2350523
ltotaltrade_usa | .1433543 .0451913 3.17 0.025 .0271863 .2595224
ltotaltrade_mca | -.0104562 .0550203 -0.19 0.857 -.1518903 .130978
ltotaltrade_row | .0377084 .0232289 1.62 0.165 -.0220033 .0974202
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