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Public Economics Lectures Part 1: Introduction

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Triple Difference<br />

Some studies use a “triple difference” (DDD)<br />

Chetty, Looney, Kroft (2009): experiment using treatment/control<br />

products, treatment/control stores<br />

DDD = DD TS − DD CS<br />

DD TS : difference of treat., cntrl products in treat. store<br />

DD CS : difference of treat., cntrl. products in cntrl. store<br />

DDD is mainly useful only as a robustness check:<br />

DD CS ̸= 0, unconvincing that DDD removes all bias<br />

DD CS = 0, then DD = DDD but DD has smaller s.e.<br />

<strong>Public</strong> <strong>Economics</strong> <strong>Lectures</strong> () <strong>Part</strong> 2: Tax Incidence 47 / 142

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