How to improve school-choice systems

In the mechanics of school admissions, some algorithms work better than others

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Alina Dizik | Jan 21, 2019

Sections Public Policy

Many large school districts have embraced school choice in recent years. Instead of assigning children to their neighborhood schools, districts have tried to make it easier for students and families to choose schools. 

But districts with centralized admissions systems use different algorithms, and some work better than others, research suggests. Some spur families to try to game the system, according to Princeton’s Adam Kapor and Christopher Neilson and Chicago Booth’s Seth Zimmerman, who encourage school districts to use what the researchers see as a better alternative. 

They studied the high-school choices of students in New Haven, Connecticut, which has used a centralized admissions system since at least 1997. Like Barcelona, Beijing, and Charlotte, North Carolina, New Haven uses an algorithm known as the Boston Mechanism, which makes it easier for families to get into schools listed first on their application. However, if families rank a popular school first, they still have little chance of being admitted—to that school or any other popular school, as in-demand schools fill with students who ranked them first. So the choice of which school to list first becomes important. 

Filling out an application on this kind of system requires families to know their chances of admission at each school. But Kapor, Neilson, and Zimmerman surveyed 417 families of rising ninth graders in 2015 and 2017 and find applicants did not necessarily know that—so lacked the information they needed to successfully navigate the process. 

Many families ranked their first-choice schools first, but their chances of admission were lower than they realized. On average, parents thought the chance of getting their child into a top-choice school was 30 percentage points higher than it really was, as determined by the algorithm.

Someone assuming that families make no mistakes in the Boston setting would conclude simplification results in worse outcomes.

Others mistakenly believed they would have an easier time getting into a school that they liked but did not think was the best fit, so about a third of them, in a misguided attempt to play the odds, demoted their first-choice school on the application. Lower-income families deployed less-accurate strategies than households with higher incomes.

“While students play strategically and attempt to trade off preference intensity against admissions chances, they do so using mistaken beliefs about admissions chances,” the researchers write.

The researchers created a model to simulate student preferences and outcomes on the basis of another mechanism for school admissions: Deferred Acceptance. In this, families rank schools in order of preference, but each choice is weighted the same and is treated in sequential order. When districts run school choice using Deferred Acceptance, families no longer need to know their admissions chances to fill out their applications correctly. The best approach is for families to write down the schools they like, in their preference order. This simplification results in better outcomes, given the mistakes families make when confronted with the more complicated Boston Mechanism, the researchers conclude.

That said, someone assuming that families make no mistakes in the Boston setting would conclude simplification results in worse outcomes, write Kapor, Neilson, and Zimmerman—explaining that this is why it is important, when designing school-choice systems, to think about the mistakes families make. 

To fully benefit from the simpler, Deferred Acceptance approach, parents must be encouraged to honestly and simply state their preferences. Districts in cities including New York, Chicago, and Boston use the Deferred Acceptance system, which the researchers argue results in greater welfare for families. 

There are ways to apply the findings beyond public-school districts, Zimmerman says, citing the process by which medical-school graduates select residencies. By telling residency applicants to simply list the schools or hospitals they like in order of preference, Zimmerman says, “you can convince most people to make the best choices, once you simplify the process.”