Regression discontinuity (RD) models are commonly used to nonparametrically identify and estimate a local average treatment eect (LATE). Dong and Lewbel (2015) show how a derivative of this LATE can be estimated. They use their Treatment Eect Derivative (TED) to estimate how the LATE would change if the RD threshold changed. We argue that their estimator should be employed in most RD applications, as a way to assess the stability and hence external validity of RD estimates. Closely related to TED, we dene the Complier Probability Derivative (CPD). Just as TED measures stability of the treatment eect, the CPD measures stability of the complier population in fuzzy designs. We provide Stata code that can be used to easily implement TED and CPD estimation, and apply it to some real data sets.
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