封面图片来源:百度。 原文信息:Doudchenko N, Imbens G W. Balancing, Regression, Difference-In-Differences and Synthetic Control Methods: A Synthesis[J]. Nber Working Papers, 2016.把DID、合成控制以及约束回归纳入一个统一的分析框架,同时作者还提出了一个新的估计方法。 作者提出的上述估计框架,在某些条件下,就可以转变为我们比较熟悉的估计方法,如:
(一)DID方法
就是最小化(1)式,但要施加上面第二个和为1的约束,第三个非负约束以及第五个常数权重约束。实际上,这几个约束一上去,不需要数据拟合,可以立马得到 ,
这样的话,若处理前只有一个时期,很容易看到平均处理效果为, 一个典型的双重差分形式。
(二)合成控制
Abadie et al.(2010)的原始论文实际上施加了三个约束:第一个无截距约束,第二个和为1约束,第三个非负约束。不过他由于使用了其他特征变量(处理个体)和(控制个体),使得(1)式稍微有点变化,即他分两步循环优化, 第一步,对于给定的权重对角矩阵V,最小化下式,
(三)约束回归
就是最小化(1)式,但是没有其他约束,譬如截距不必为0,权重不必为正、其和不必为1等。
(四)最优子集选择
他就是从一堆控制个体中选择部分个体进入(1)式优化。数学上,可以写为,
这个模型的调和参数就k,就是选几个控制个体进入模型。实际上很多DID 就是非正式地选择了某个或某几个控制个体进入模型,不如上述方法规范。 In a seminal paper Abadie et al (2010) develop the synthetic control procedure for estimating the effect of a treatment, in the presence of a single treated unit and a number of control units, with pre-treatment outcomes observed for all units. The method constructs a set of weights such that covariates and pre-treatment outcomes of the treated unit are approximately matched by a weighted average of control units. The weights are restricted to be nonnegative and sum to one, which allows the procedure to obtain the weights even when the number of lagged outcomes is modest relative to the number of control units, a setting that is not uncommon in applications. In the current paper we propose a more general class of synthetic control estimators that allows researchers to relax some of the restrictions in the ADH method. We allow the weights to be negative, do not necessarily restrict the sum of the weights, and allow for a permanent additive difference between the treated unit and the controls, similar to difference-in-difference procedures. The weights directly minimize the distance between the lagged outcomes for the treated and the control units, using regularization methods to deal with a potentially large number of possible control units. 亲爱的读者,如果您从阅读本文中得到启发,或者受益,请您为本文打赏,以感谢推文者的辛苦工作,鼓励她(他)下一期提供更精彩的推文(香樟打赏直接给每期的推文作者)。推文仅代表文章原作者观点及推文作者的评论观点,并不代表香樟经济学术圈公众号平台观点。香樟致力于提供学术研究公共品,对香樟最好的回馈就是向平台赐稿。联系邮箱cectuiwen@163.com.
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