求大神解释，在做豪斯曼检验时出现了这样一段话，请问是什么意思？
the rank of the differenced variance matrix (3) 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.
部分数据是这样的：

[url=][/url]