Introduction to theJBESSymposium on Program and Policy Evaluation
本文是《商业与经济统计杂志》关于项目与政策评估专题的导言，介绍一组利用州或省政策随时间变化来识别政策效应的文章，适合对政策评估方法感兴趣的学者快速了解该专题内容。
Meyer. Each of the articles in this group relies on changes in state (or provincial) policies over time to identify the effect of policy changes. States instituting reforms constitute the treatment group, whereas nonreforming states are assumed to provide a valid control group. The treatment-control parison is made after differencing state observations across years. For example, in their article on Canadian national health insurance, Jon Gruber and Maria Hanratty use the fact that Canadian provinces introduced national health insurance at different times to estimate the effect of national health insurance on changes in labor-market outcomes. The presumption is that contemporaneous changes in neighboring provinces provide a control for labor-market trends that are unrelated to health-care reform. Similarly, in their article on minimum wages, Taeil Kim and Lowell Taylor use the variation afforded by California's increase in the state minimum wage in the late 1980s. The presumption underlying this work is that, within narrowly defined sectors engaged in retail trade, changes in employ