```stata
xtreg pack_sales_per_capita price_per_pack i.year, fe vce(cluster state_num) # Same as vce(robust) for xtreg
Fixed-effects (within) regression Number of obs = 2,550
Group variable: state_num Number of groups = 51
R-sq: Obs per group:
within = 0.8513 min = 50
between = 0.2297 avg = 50.0
overall = 0.6393 max = 50
F(50,50) = 29393.64
corr(u_i, Xb) = 0.0329 Prob > F = 0.0000
(Std. Err. adjusted for 51 clusters in state_num)
--------------------------------------------------------------------------------
| Robust
pack_sales_p~a | Coef. Std. Err. t P>|t| [95% Conf. Interval]
---------------+----------------------------------------------------------------
price_per_pack | -6.625379 2.021698 -3.28 0.002 -10.68608 -2.564679
---------------+----------------------------------------------------------------
sigma_u | 21.173219
sigma_e | 13.8135
rho | .70144358 (fraction of variance due to u_i)
--------------------------------------------------------------------------------
```
> [!info]- Last updated: February 3, 2025