yit =a+xiB+uit i=1,.,n t=1,.,T ·情形4：不讨论。Because it is seldom meaningful to ask if the intercepts are the same when the slopes are unequal,we shall ignore the type of restrictions. • 情形4：不讨论。 Because it is seldom meaningful to ask if the intercepts are the same when the slopes are unequal, we shall ignore the type of restrictions. i = 1,  , n t = 1,  ,T i t i t i ui t y = + x  +