AI 中文总结
本文针对双重差分法中部分队列平行趋势不成立的问题，提出估计仅针对可信队列的局部平均处理效应，结合诚实敏感性边界，通过模拟和实证验证了其有效性。
AI 中文摘要
当部分处理队列满足平行趋势假设而另一部分不满足时，所有队列的平均处理效应（ATT）这一整体目标将难以准确估计。本文提出改变估计量而非维护原估计量：可信子群体局部平均处理效应（LATT）是仅针对满足平行趋势的队列子群体的效应，仅需所选队列满足平行趋势这一比ATT要求更弱的条件，当ATT的平行趋势不成立时该条件仍可能成立。该方法通过对标准组-时间效应按队列加权来估计，同时结合诚实敏感性边界处理预趋势检验无法排除的剩余违反情况。模拟结果证实，当预趋势对处理后违反情况的预测性越强，该方法的优势越显著。在一项应用中，页岩气繁荣对当地房价的合并显著正向估计依赖于 onset 前已呈现趋势的队列，而可信子群体的估计显示无效应。
英文摘要
When parallel trends fails for some treated cohorts but not others, the average treatment effect on the treated (ATT), an average over all of them, is exactly the target that becomes hard to recover. We propose changing the estimand rather than defending it. The credible-subpopulation local ATT (LATT) is the effect for the subpopulation of cohorts whose parallel trends is credible, and it is point-identified under parallel trends for the selected cohorts alone, a weaker requirement that can hold when the ATT's fails. It is estimated by reweighting standard group-time effects toward those cohorts, and paired with honest sensitivity bounds on the residual violation that a pre-trend screen cannot rule out. The method's advantage grows with how informative pre-trends are about post-treatment violations, as simulations confirm. In an application, a significantly positive pooled estimate of the shale boom's effect on local house prices proves to rest on cohorts already trending before onset, and the credible subpopulation reveals no effect.
