報導指出,從2019年開始,北京語言大學就禁止哈佛的學生在七月四日聚會慶祝美國國慶,類似的不友善行為最終讓哈佛決定變更合作對象。對於課程遷移,哈佛大學中國研究中心主任柯偉林 (William C. Kirby) 表示,課程遷移完全出於後勤因素,哈佛大學的其他機構並沒有要終止合作計畫,也仍在試著深化與中國之間的合作關係。
對於經濟學獎附著在諾貝爾獎,經濟學界有兩極看法,贊成者認為附著在諾貝爾獎可提升經濟學獎聲望及價值。反對者則認為不倫不類有高攀諾貝爾獎之嫌,其中反對最力者首推擁有奧地利與英國雙國籍的經濟學家佛里德利希-海耶克 (Friedrich August von Hayek),1969 年首次頒獎日,他曾發表公開演說反對經濟學獎附生於諾貝爾獎。諷刺的是 1974 年得獎人就是海耶克,而他也大大方方的上台領獎,畢竟這個獎太不同凡響了。
How can we use a natural experiment – situations arising in real life that resemble randomised experiments – to examine whether additional years of education affect future income?
2021 economic sciences laureate Joshua Angrist and his colleague Alan Krueger (now deceased) showed how this could be done in a landmark article. In the US, children can leave school when they turn 16 or 17, depending on the state where they go to school. Because all children who are born in a particular calendar year start school on the same date, children who are born early in the year can leave school sooner than children born later in the year. When Angrist and Krueger compared people born in the first and fourth quarters of the year, they saw that the first group had, on average, spent less time in education. People born in the first quarter also had lower incomes than those born in the fourth quarter. As adults they thus had both less education and lower incomes than those born late in the year.
Because chance decides exactly when a person is born, Angrist and Krueger were able to use this natural experiment to establish a causal relationship showing that more education leads to higher earnings: the effect of an additional year of education on income was nine per cent. It was surprising that this effect was stronger than the association between education and income, which amounted to seven per cent. If ambitious and intelligent people have both high levels of education and high incomes (regardless of education) the result should have been the opposite; the correlation should have been stronger than the causal relationship.
The 2021 Sveriges Riksbank Prize in Economic Sciences in Memory of Alfred Nobel has been awarded with one half to David Card “for his empirical contributions to labour economics” and the other half jointly to Joshua D. Angrist and Guido W. Imbens “for their methodological contributions to the analysis of causal relationships.”
This year’s economic sciences laureates have shown that it is possible to answer questions in the social sciences using natural experiments. The key is to use situations in which chance events or policy changes result in groups of people being treated differently, in a way that resembles clinical trials in medicine.
We can imagine a natural experiment as if it randomly divides individuals into a treatment group and a control group. The treatment group is entitled to participate in a programme while the control group is not. This year’s economic sciences laureates Joshua Angrist and Guido Imbens showed that it is possible to estimate the effect of the programme by applying a two-step process (known as the instrumental variables method). The first step investigates how the natural experiment affects the probability of programme participa¬tion. The second step then considers this probability when evaluating the effect of the actual pro¬gramme. Given a few assumptions, which Imbens and Angrist formulated and discussed in detail, the researchers can thus estimate the impact of the programme, even when there is no information about who was actually affected by the natural experiment. One important conclusion is that it is only possible to estimate the effect among the people who changed their behaviour as a result of the natural experiment. It is not possible to determine which individuals are included in this group, but we can determine its size. The effect for this group has been named the local average treatment effect, LATE.
Joshua Angrist and Guido Imbens thus showed exactly what conclusions about cause and effect can be drawn from natural experiments. Their analysis is also relevant for randomised experiments where we do not have complete control over who participates in the intervention, which is the case in almost all field experiments. The framework developed by Angrist and Imbens has been widely adopted by researchers who work with observational data. By clarifying the assumptions necessary to establish a causal relationship, their framework has also increased the transparency – and thus credibility – of empirical research.
The 2021 Sveriges Riksbank Prize in Economic Sciences in Memory of Alfred Nobel has been awarded with one half to David Card “for his empirical contributions to labour economics” and the other half jointly to Joshua D. Angrist and Guido W. Imbens “for their methodological contributions to the analysis of causal relationships.”
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