| 来源类型 | Working Paper |
| 规范类型 | 报告 |
| DOI | 10.3386/t0312 |
| 来源ID | Technical Working Paper 0312 |
| Inference with \"Difference in Differences\" with a Small Number of Policy Changes | |
| Timothy Conley; Christopher Taber | |
| 发表日期 | 2005-07-11 |
| 出版年 | 2005 |
| 语种 | 英语 |
| 摘要 | Difference in differences methods have become very popular in applied work. This paper provides a new method for inference in these models when there are a small number of policy changes. This situation occurs in many implementations of these estimators. Identification of the key parameter typically arises when a group "changes" some particular policy. The asymptotic approximations that are typically employed assume that the number of cross sectional groups, N, times the number of time periods, T, is large. However, even when N or T is large, the number of actual policy changes observed in the data is often very small. In this case, we argue that point estimators of treatment effects should not be thought of as being consistent and that the standard methods that researchers use to perform inference in these models are not appropriate. We develop an alternative approach to inference under the assumption that there are a finite number of policy changes in the data, using asymptotic approximations as the number of non-changing groups gets large. In this situation we cannot obtain a consistent point estimator for the key treatment effect parameter. However, we can consistently estimate the finite-sample distribution of the treatment effect estimator, up to the unknown parameter itself. This allows us to perform hypothesis tests and construct confidence intervals. For expositional and motivational purposes, we focus on the difference in differences case, but our approach should be appropriate more generally in treatment effect models which employ a large number of controls, but a small number of treatments. We demonstrate the use of the approach by analyzing the effect of college merit aide programs on college attendance. We show that in some cases the standard approach can give misleading results. |
| URL | https://www.nber.org/papers/t0312 |
| 来源智库 | National Bureau of Economic Research (United States) |
| 引用统计 | |
| 资源类型 | 智库出版物 |
| 条目标识符 | http://119.78.100.153/handle/2XGU8XDN/569114 |
| 推荐引用方式 GB/T 7714 | Timothy Conley,Christopher Taber. Inference with \"Difference in Differences\" with a Small Number of Policy Changes. 2005. |
| 条目包含的文件 | ||||||
| 文件名称/大小 | 资源类型 | 版本类型 | 开放类型 | 使用许可 | ||
| t0312.pdf（649KB） | 智库出版物 | 限制开放 | CC BY-NC-SA | 浏览 | ||
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