做豪斯曼检验时出现note语句
Note: the rank of the differenced variance matrix (4) does not equal the number of
        coefficients being tested (6); be sure this is what you expect, or there may be
        problems computing the test.  Examine the output of your estimators for
        anything unexpected and possibly consider scaling your variables so that the
        coefficients are on a similar scale

        Coefficients ----
        (b)          (B)            (b-B)     sqrt(diag(V_b-V_B))
        FE           RE         Difference          S.E.
       
agdp        -2.55e-06    -6.24e-07       -1.93e-06        4.00e-07
hlw        .0035734     .0056287       -.0020553        .0005706
uri        -5.20e-06    -1.79e-06       -3.40e-06        6.57e-07
hm        .0008345    -.0051603        .0059947        .0031061
el        -.0130077    -.0070274       -.0059803        .0018962
_cons        .6096597     .3915511        .2181086        .0523883
       
        b = consistent under Ho and Ha; obtained from xtreg
B =        inconsistent under Ha, efficient under Ho; obtained from xtreg

Test:  Ho:        difference in coefficients not systematic

        chi2(4) = (b-B)'[(V_b-V_B)^(-1)](b-B)
        =       32.51
        Prob>chi2 =      0.0000
        (V_b-V_B is not positive definite)