中文摘要
我们实证考察了在固定人群中、当该人群遭受总体冲击时，具有内部有效性的因果效应微观估计在时间上的可推广性。利用面板数据，我们表明，印度和加纳的农业投资回报、斯里兰卡中小型非农企业的投资回报以及印度尼西亚的教育回报在不同时期之间显著波动。我们展示了这些投资回报如何与具体的、可度量的且具有经济相关性的总体冲击相互作用，重点关注降雨和价格波动。我们还基于两个农业样本中降雨冲击分布参数的估计，得到了回报置信区间的下界估计。我们发现，即便是这些下界置信区间，也显著宽于研究中通常仅基于抽样误差所给出的置信区间，而后者大多基于单一年度样本。我们还发现，降雨的横截面变异无法被可靠地用于复制总体内部的降雨变异性。基于我们的发现，我们讨论了将外部冲击信息纳入政策回报评估的方法。
Abstract
Abstract We examine empirically the generalizability of internally valid micro-estimates of causal effects in a fixed population over time when that population is subject to aggregate shocks. Using panel data, we show that the returns to investments in agriculture in India and Ghana, small and medium non-farm enterprises in Sri Lanka, and schooling in Indonesia fluctuate significantly across time periods. We show how the returns to these investments interact with specific, measurable, and economically relevant aggregate shocks, focusing on rainfall and price fluctuations. We also obtain lower-bound estimates of confidence intervals of the returns based on estimates of the parameters of the distributions of rainfall shocks in our two agricultural samples. We find that even these lower-bound confidence intervals are substantially wider than those based solely on sampling error that are commonly provided in studies, most of which are based on single-year samples. We also find that cross-sectional variation in rainfall cannot be confidently used to replicate within-population rainfall variability. Based on our findings, we discuss methods for incorporating information on external shocks into evaluations of the returns to policy.