xtabond2 depvar varlist [if exp] [in range] [weight] [, level(#) twostep robust cluster(varname) noconstant small noleveleq orthogonal gmmopt [gmmopt ...]   ivopt [ivopt ...] artests(#)  arlevels h(#) nodiffsargan nomata]
  上面是xtabond2的语法格式，下面是给出的几个例子：
    . xtabond2 n l.n l(0/1).(w k) yr1980-yr1984, gmm(l.n w k)  iv(yr1980-yr1984, passthru) noleveleq small
    . xtabond2 n l.n l(0/1).(w k) yr1980-yr1984, gmm(l.n w k)  iv(yr1980-yr1984, mz) robust twostep small h(2)
    . xtabond2 n l(1/2).n l(0/1).w l(0/2).(k ys) yr1980-yr1984, gmm(l.n w k)  iv(yr1980-yr1984) robust twostep small
我想问的是yr1980-yr1984,这些都是时间的虚拟变量，为何放在iv()中?
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