求助各位大神做豪斯曼检验的时候为什么会这样
Note: the rank of the differenced variance matrix (6) does not equal the number of coefficients being tested (7); 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.