在对面板数据固定效应和随机效应模型进行选择时（经检验，两个模型均优于ols模型），进行hausman检验时，出现“the rank of the differenced variance matrix (9) does not equal the number of coefficients being tested (10); 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.”这样的提示，请问是什么原因引起的？如何解决？