求助大神们，固定效应中加入行业虚拟变量而且也drop掉了第一个行业，但是为什么回归时显示剩余的行业虚拟变量全部被omitted呢？（数据放在附件了）
我的代码及结果：. xtset company year panel variable:company (unbalanced)
time variable:year, 2011 to 2016
delta:1 unit

. tab indus_sw,gen(ind_sw)

indus_sw | Freq. Percent Cum.
------------+-----------------------------------
采掘 | 18 0.53 0.53
传媒 | 120 3.52 4.05
电气设备 | 306 8.98 13.02
电子 | 311 9.12 22.15
房地产 | 6 0.18 22.32
纺织服装 | 168 4.93 27.25
钢铁 | 24 0.70 27.96
公用事业 | 30 0.88 28.84
国防军工 | 35 1.03 29.86
化工 | 389 11.41 41.27
机械设备 | 388 11.38 52.65
计算机 | 198 5.81 58.46
家用电器 | 137 4.02 62.48
建筑材料 | 132 3.87 66.35
建筑装饰 | 102 2.99 69.35
交通运输 | 18 0.53 69.87
农林牧渔 | 96 2.82 72.69
汽车 | 185 5.43 78.12
轻工制造 | 191 5.60 83.72
商业贸易 | 12 0.35 84.07
食品饮料 | 84 2.46 86.54
通信 | 102 2.99 89.53
休闲服务 | 6 0.18 89.70
医药生物 | 240 7.04 96.74
有色金属 | 111 3.26 100.00
------------+-----------------------------------
Total | 3,409 100.00

.
. drop ind_sw1

. xtreg y x1 x2 c.c_x1#c.c_x2 x3 x4 x5 x6 x7 ind_sw*,fe
note: ind_sw2 omitted because of collinearity
note: ind_sw3 omitted because of collinearity
note: ind_sw4 omitted because of collinearity
note: ind_sw5 omitted because of collinearity
note: ind_sw6 omitted because of collinearity
note: ind_sw7 omitted because of collinearity
note: ind_sw8 omitted because of collinearity
note: ind_sw9 omitted because of collinearity
note: ind_sw10 omitted because of collinearity
note: ind_sw11 omitted because of collinearity
note: ind_sw12 omitted because of collinearity
note: ind_sw13 omitted because of collinearity
note: ind_sw14 omitted because of collinearity
note: ind_sw15 omitted because of collinearity
note: ind_sw16 omitted because of collinearity
note: ind_sw17 omitted because of collinearity
note: ind_sw18 omitted because of collinearity
note: ind_sw19 omitted because of collinearity
note: ind_sw20 omitted because of collinearity
note: ind_sw21 omitted because of collinearity
note: ind_sw22 omitted because of collinearity
note: ind_sw23 omitted because of collinearity
note: ind_sw24 omitted because of collinearity
note: ind_sw25 omitted because of collinearity

Fixed-effects (within) regression Number of obs = 3409
Group variable: company Number of groups = 570

R-sq:within= 0.0866 Obs per group: min = 5
between = 0.0034 avg = 6.0
overall = 0.0066 max = 6

F(8,2831) = 33.57
corr(u_i, Xb)= -0.3911 Prob > F = 0.0000

-------------------------------------------------------------------------------
y | Coef. Std. Err. t P>|t| [95% Conf. Interval]
--------------+----------------------------------------------------------------
x1 | 1.39e-12 5.10e-13 2.72 0.007 3.87e-13 2.39e-12
x2 |-.0004315 .0001572 -2.75 0.006 -.0007397 -.0001233
|
c.c_x1#c.c_x2 | 6.36e-13 3.90e-13 1.63 0.103 -1.29e-13 1.40e-12
|
x3 |-.0084076 .0005553 -15.14 0.000 -.0094963 -.0073189
x4 | .0110672 .0041595 2.66 0.008 .0029112 .0192233
x5 | .0009815 .000522 1.88 0.060 -.000042 .002005
x6 |-.0066004 .0024292 -2.72 0.007 -.0113636 -.0018373
x7 | .0013011 .0001313 9.91 0.000 .0010436 .0015587
ind_sw2 | 0(omitted)
ind_sw3 | 0(omitted)
ind_sw4 | 0(omitted)
ind_sw5 | 0(omitted)
ind_sw6 | 0(omitted)
ind_sw7 | 0(omitted)
ind_sw8 | 0(omitted)
ind_sw9 | 0(omitted)
ind_sw10 | 0(omitted)
ind_sw11 | 0(omitted)
ind_sw12 | 0(omitted)
ind_sw13 | 0(omitted)
ind_sw14 | 0(omitted)
ind_sw15 | 0(omitted)
ind_sw16 | 0(omitted)
ind_sw17 | 0(omitted)
ind_sw18 | 0(omitted)
ind_sw19 | 0(omitted)
ind_sw20 | 0(omitted)
ind_sw21 | 0(omitted)
ind_sw22 | 0(omitted)
ind_sw23 | 0(omitted)
ind_sw24 | 0(omitted)
ind_sw25 | 0(omitted)
_cons | .1866912 .0106203 17.58 0.000 .165867 .2075155
--------------+----------------------------------------------------------------
sigma_u |.01884253
sigma_e |.00833753
rho |.83626546 (fraction of variance due to u_i)
-------------------------------------------------------------------------------
F test that all u_i=0: F(569, 2831) = 24.63 Prob > F = 0.0000

.