如题，求：PPML估计的方法简介，使用情况及stata操作。1、在什么情况下用这种估计方法
2、stata 操作中

ppml estimation (the dummy z is correctly dropped, the observations with
    z=1 are also dropped)

        . ppml y x w z

    ppml estimation with the keep option (the dummy z is correctly dropped but
    no observations are dropped; this is equivalent to not conditioning on z)

        . ppml y x w z, k

    ppml estimation (v is correctly dropped, no observations are dropped
    because v is not a dummy)

        . ppml y x w v
都是在什么情况下用。