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更多“鸟枪法试验（shotgun experiment)”相关的问题
(i)利用一个2SLS程序估计
其中，sibs是educ的IV。以通常形式报告结果。
(iii)现在，使用如下两步法，它不仅得到βi不一致的参数估计值，以及不那么一致的标准误。第一步，将educi仅对sibsi作回归，并得到拟合值educi。(注意，这是一个不正确的第一阶段回归。)然后，在第二步，做log(wagei)对educi、experi、terurei和balcki，i=1，···，n的回归。从这个不正确的两步法中得到的教育回报估计值与正确的2SLS估计值相比如何？
On September 27th a 53-year-old petty criminal, Duane Morrison, walked into a school in Bailey, Colorado, with two guns. He took six girls hostage, molested some of them, and killed one before committing suicide as police stormed the room.
And on September 29th a boy brought two guns into his school in Cazenovia, Wisconsin. Prosecutors say that 15-year-old Eric Hainstock may have planned to kill several people. But staff acted quickly when they saw him with a shotgun, calling the police and putting the school into "lock-down". The head teacher, who confronted him in a corridor, was the only one killed.
October 2nd a 32-year-old milk-truck driver, Charles Roberts, entered a one-room Amish school in Nickel Mines, Pennsylvania. He lined the girls up, tied their feet and, after an hour, shot them, killing at least five. He killed himself as police broke into the classroom.
What to make of such horrors？ Some experts see the Colorado and Pennsylvania cases as an extreme manifestation of a culture of violence against women. Both killers appeared to have a sexual motive, and both let all the boys in the classroom go free. But it is hard to infer from such unusual examples, and one must note that violence against women is less than half what it was in 1995.
Other experts see all three cases as symptomatic of a change in the way men commit suicide. Helen Smith, a forensic psychologist, told a radio audience "men are deciding to take their lives, "and they're not going alone anymore. They're taking people down with them". True, but not very often.
Gun-control enthusiasts think school massacres show the need for tighter restrictions. It is too easy, they say, for criminals such as Mr. Morrison and juveniles such as Mr. Hainstock to obtain guns. Gun enthusiasts draw the opposite conclusion: that if more teachers carried concealed handguns, they could shoot potential child-killers before they kill.
George Bush has now called for a conference on school violence. Will it unearth anything new, or valuable？ After the Columbine massacre in 1999, the FBI produced a report on school shooters. It concluded that it was impossible to draw up a useful profile of a potential shooter because "a great many adolescents who will never commit violent acts will show some of the behaviours on any checklist of warning signs".
According to the passage, an epidemic of violence in American classrooms was inevitable in that ______.
A．three school shootings in six days make a trend.
B．children are less likely to be murdered outside the school walls than within them.
C．there is no limits to get a gun for children.
D．an epidemic of violence is not only in American classrooms but also in society.
