b = consistent under Ho and Ha; obtained from xtreg
B = inconsistent under Ha, efficient under Ho; obtained from xtreg

Test:  Ho:  difference in coefficients not systematic

                  chi2(6) = (b-B)'[(V_b-V_B)^(-1)](b-B)
                          =     6767.46
                Prob>chi2 =      0.0000
                (V_b-V_B is not positive definite)

请问(V_b-V_B is not positive definite)什么意思，对选定结果有影响吗？这里还是用固定吗？新手谢过。