我做了固定效应和随机效应的hausman检验，出现了Note: the rank of the differenced variance matrix (98) does not equal the number of coefficients
        being tested (118); 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.


求大神解释这个note