有谁能解答一下在对面板进行“xtpcse 变量 countryi- countryj t,corr(ar1)”程序运算时，为什么会出现以下的结果啊？
Number of gaps in sample:  7
(note: computations for rho restarted at each gap)
no time periods are common to all panels, cannot estimate disturbance
covariance matrix using casewise inclusion
r(459);
请高手解答，万分感谢！急求！