stata 为什么probit估计之后，变量的边际效应与估计系数完全一样？-经管之家官网！

请教各位，做probit回归后，希望得到各变量在均值处的边际效应，输入命令后，结果显示个变量的边际效应与估计系数完全一样。请问这是怎么回事？如何得出各变量的边际效应？命令及结果如下所示。
命令：xtprobit newproductyummy lnepu lnepustateown lntotalassetpriceadjust age profitrate exportintensity financingrestriction fixassetratio exportratio
结果：
Random-effects probit regression Number of obs = 324197
Group variable: pro Number of groups = 41271
Random effects u_i ~ Gaussian Obs per group: min = 1
avg = 7.9
max = 8
Wald chi2(9) = 5859.24
Log likelihood= -66364.739 Prob > chi2 = 0.0000
-----------------------------------------------------------------------------------------
newproductyummy | Coef. Std. Err. z P>|z| [95% Conf. Interval]
------------------------+----------------------------------------------------------------
lnepu |-.3198235 .0169239 -18.90 0.000 -.3529937 -.2866533
lnepustateown | .0487838 .0045945 10.62 0.000 .0397788 .0577888
lntotalassetpriceadjust | .5515403 .0077518 71.15 0.000 .5363471 .5667335
age | .0002782 .0000619 4.49 0.000 .0001569 .0003996
profitrate | .0136898 .0047137 2.90 0.004 .0044512 .0229284
exportintensity | .2553192 .0951012 2.68 0.007 .0689243 .4417142
financingrestriction |-.0003315 .0032297 -0.10 0.918 -.0066615 .0059985
fixassetratio |-.6449095 .0412537 -15.63 0.000 -.7257652 -.5640538
exportratio |-.0213164 .0928364 -0.23 0.818 -.2032724 .1606396
_cons |-6.860992 .1086093 -63.17 0.000 -7.073862 -6.648122
------------------------+----------------------------------------------------------------
/lnsig2u | 1.547799 .0149549 1.518488 1.57711
------------------------+----------------------------------------------------------------
sigma_u | 2.168204 .0162127 2.13666 2.200215
rho | .8245956 .002163 .8203157 .8287948
-----------------------------------------------------------------------------------------
Likelihood-ratio test of rho=0: chibar2(01) =7.0e+04 Prob >= chibar2 = 0.000
命令： margins,dydx(*) atmean
结果：
Conditional marginal effects Number of obs = 324197
Model VCE : OIM
Expression : Linear prediction, predict()
dy/dx w.r.t. : lnepu lnepustateown lntotalassetpriceadjust age profitrate exportintensity
financingrestriction fixassetratio exportratio
at : lnepu = 4.44505 (mean)
lnepustate~n = 1.447035 (mean)
lntotalass~t = 9.648673 (mean)
age = 21.56639 (mean)
profitrate = -.0366111 (mean)
exportinte~y = .196872 (mean)
financingr~n = .0628949 (mean)
fixassetra~o = .3837683 (mean)
exportratio = .1937937 (mean)
-----------------------------------------------------------------------------------------
| Delta-method
| dy/dx Std. Err. z P>|z| [95% Conf. Interval]
------------------------+----------------------------------------------------------------
lnepu |-.3198235 .0169239 -18.90 0.000 -.3529937 -.2866533
lnepustateown | .0487838 .0045945 10.62 0.000 .0397788 .0577888
lntotalassetpriceadjust | .5515403 .0077518 71.15 0.000 .5363471 .5667335
age | .0002782 .0000619 4.49 0.000 .0001569 .0003996
profitrate | .0136898 .0047137 2.90 0.004 .0044512 .0229284
exportintensity | .2553192 .0951012 2.68 0.007 .0689243 .4417142
financingrestriction |-.0003315 .0032297 -0.10 0.918 -.0066615 .0059985
fixassetratio |-.6449095 .0412537 -15.63 0.000 -.7257652 -.5640538
exportratio |-.0213164 .0928364 -0.23 0.818 -.2032724 .1606396
-----------------------------------------------------------------------------------------
边际效应与估计系数完全一致，是否与结果中的Expression: Linear prediction, predict()有关。我看到的关于margins的例子中，少有Expression: Linear prediction, predict()，一般是Expression : Pr(grade), predict()这类结果。
多谢！

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