Hi everyone!

I have a data set that I enclosed here. My dependent variable is tariff. The variable of interest is import volume. I also include country fix effect and industry fix effect, and run the following estimation:

areg tariff c.import_volume#i.country_FE i.country_FE,a(HS_2digits_FE) robust

I got the result but the standard error for the coefficient of country 5 is missing. Moreover, when I repeat the regression again, the coefficient of this country changes while the coefficients of other countries do not. I enclosed one example result I got here. Could anyone help me to explain it? Thanks in advance.


. areg concession c.import_volume#i.country_FE i.country_FE,a(HS_2digits_FE) rob
> ust

Linear regression, absorbing indicators Number of obs = 4,913
Absorbed variable: HS_2digits_FE No. of categories = 9
F( 24, 4879) = .
Prob > F = .
R-squared = 0.5895
Adj R-squared = 0.5868
Root MSE = 5.4004

-------------------------------------------------------------------------------
| Robust
Tariff | Coef. Std. Err. t P>|t| [95% Conf. Interval]
--------------+----------------------------------------------------------------
country_FE#|
c. |
import_volume |
1 | -2.51614 .7682496 -3.28 0.001 -4.022255 -1.010025
2 | -6.510891 25.76246 -0.25 0.800 -57.0169 43.99512
3 | -.2954423 .1289958 -2.29 0.022 -.5483322 -.0425524
4 | -.1119799 .0343651 -3.26 0.001 -.179351 -.0446088
5 | 2.10e-13 . . . . .
6 | -.0399625 .0273511 -1.46 0.144 -.093583 .0136581
7 | .0577411 .0194236 2.97 0.003 .0196622 .09582
8 | -.7764847 1.053377 -0.74 0.461 -2.841577 1.288608
9 | -505.2796 84.64939 -5.97 0.000 -671.2306 -339.3287
10 | -.0067254 .003443 -1.95 0.051 -.0134752 .0000243
11 | .0659294 .0563759 1.17 0.242 -.0445928 .1764516
12 | -.0049848 .0068925 -0.72 0.470 -.0184972 .0085276
13 | .0030149 .0008714 3.46 0.001 .0013065 .0047232
|
country_FE |
2 | 4.012442 .3934771 10.20 0.000 3.24105 4.783834
3 | 8.091445 .2673583 30.26 0.000 7.567303 8.615588
4 | 5.947518 .55061 10.80 0.000 4.868074 7.026961
5 | 8.669858 .4818935 17.99 0.000 7.725129 9.614586
6 | 17.64244 .9150317 19.28 0.000 15.84857 19.43632
7 | -2.62176 .1529011 -17.15 0.000 -2.921515 -2.322005
8 | 5.822024 2.54128 2.29 0.022 .8399706 10.80408
9 | 5.434124 .7494499 7.25 0.000 3.964865 6.903384
10 | -.6122702 .1252803 -4.89 0.000 -.857876 -.3666644
11 | 19.48841 .7780981 25.05 0.000 17.96299 21.01383
12 | 4.289819 .3765841 11.39 0.000 3.551545 5.028094
13 | 1.822885 .4191367 4.35 0.000 1.001188 2.644581
|
_cons | 3.271722 .1221558 26.78 0.000 3.032242 3.511203
-------------------------------------------------------------------------------