Quasi-Difference in Differences with Panel Data: Complete Pairing and Linear-Model LSE
2012/01/10
研討會日期 : 2012-01-10
時間 : 14:30
主講人 : Professor Myoung-Jae Lee
地點 : Conference Room C103
演講者簡介 : Myoung-Jae Lee got his Ph.D. in Economics from University of Wisconsin-Madison in 1989. He is currently serving as Professor of Economics at Korea University. His research field is in econometrics and statistics, and labor and health economics.
演講摘要 : With one treated and one untreated periods, difference in differences (DD) requires a ‘zero selection-effect (or the same time-effect) condition' that the untreated response changes be the same across the treatment and control groups, if the treatment were withheld contrary to the fact. A natural way to check out the plausibility of the condition is to backtrack one period and examine the response changes in two pre-treatment periods. If the condition holds in the pre-treatment periods, then one would go ahead and apply DD; otherwise, the pre-treatment discrepancy/gap should be accounted for in modifying DD. Under the assumption that the pre-treatment gap is persistent, the modified DD takes the form of ‘quasi-difference in differences (QD)' that may be also called ‘triple differences'. QD generalizes DD with a weaker identification condition: a time-constant, but not necessarily zero, selection effect. This paper addresses how to test whether the pre-treatment gap is persistent or not, and how to modify DD with QD accordingly. Nonparametric estimators/tests and panel linear-model-based LSE/tests are explored. A simulation study is presented to show that the proposed estimators/tests work as they are supposed to.
