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Regression Discontinuity - Institute for Policy Research

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Fallible RD?<br />

• When we probed the New Jersey data, we found that the<br />

discontinuity in the McCrary test was driven by cases with a<br />

September 1 st cutoff.<br />

• Five years ago, children who were born on September 1 st<br />

were born on a Monday and children who were born on<br />

August 31 st , were born on Sunday.<br />

• Birth record data shows that there are large “day” effects<br />

(i.e. more births on Fridays and Mondays, fewer births on<br />

weekends and holidays), so the discontinuity may be driven<br />

by day effects<br />

• Not a problem if the “day effects” are not correlated with<br />

students’ achievement, but still under investigation.<br />

• Not a problem in 4 other states.

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