The birthday cutoff for starting school

Question

A district requires that a child be 5 years old by September 1 to start school that fall. So a child born on August 31 starts a year earlier than a child born on September 2, even though they are almost the same age. You want the effect of starting school a year older on early reading scores.

Explain how the birthday cutoff enables a regression discontinuity design, why it is nearly random, the key assumption, and the limitation.

Accepted Answer

The cutoff creates a sharp jump in school-starting age. Children born just before September 1 start school having only just turned 5, while children born just after start the following year, at nearly 6. Two children born a few days apart, one on August 31 and one on September 2, are essentially the same age, yet the rule sends them to school a full year apart in starting age. That abrupt difference at the birthdate cutoff is what the design exploits. Why it is nearly random near the cutoff. Whether a child is born on August 31 or September 2 is, for the vast majority of families, a matter of…

The birthday cutoff for starting school

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