Differences in Differences (using Stata)

Differences-in-Differences

(v. 3.3)

Oscar Torres-Reyna

otorres@princeton.edu

August 2015



Intro

Differences-in-Differences regression (DID) is used to asses the causal effect of an event by comparing the set of units where the event happened (treatment group) in relation to units where the event did not happen (control group).

The logic behind DID is that if the event never happens, the differences between treatment and control groups should stay the same overtime, see graph next slide.*

*See:

OTR

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Source:

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= 0 + 1 + 2 + 3 +

Source:

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Intro

This document shows how to perform difference-in-differences regression in the following two situations:

? Event happened at the same time for all treated groups.

? Event is staggered across groups.

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