. hausman fe11 .

Note: the rank of the differenced variance matrix (1) 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.

在做hausman检验时出现以上提示，哪位高人知道是什么意思，怎么处理？请帮忙。