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Each year newly minted doctors and potential sorority sisters go through a similar recruitment process.

Sorority recruitment and residency matching programs are examples of two-sided matching markets that require a complex algorithm to match people according to their preferences.

In the case of recruitment, the chapters represent one side of the market and the potential new members represent the other.

Hundreds of sororities across North America use this algorithm.

“The chapters prioritize which women they want to invite back, just like women are prioritizing which chapters they’d like to go back to,” said Julie Johnson, national chairperson of the College Panhellenics Commitee for the National Panhellenic Conference.

A related but different computerized matching algorithm is used to place medical students into hospital residency programs, and works through the National Resident Matching Program, a non-profit that helps match medical students with a hospital. Last year, the NRMP worked with 4,427 programs across the country.

According to Mona Signer, executive director of NRMP, applicants visit the NRMP website and enter their rank order list of preferences, and the matching process occurs at the same time as applicants are applying and interviewing for positions.

Signer also said that NRMP does not replace the application process to residency programs, but rather helps both hospitals and applicants understand their best options before making a decision.

But in the past, sorority recruitment and residency placement did not need such a complicated technology.

In the 19th century, Greek organizations were only allowed to recruit college seniors, but the organizations soon began recruiting earlier and earlier to get the best new members. This “rush” for new members eventually became so chaotic that Greek organizations were recruiting new members before they entered college.

Alvin Roth, professor of economics at Stanford University — who has won a Nobel prize along with Lloyd Shapley for his work on these type of algorithms — explained this “rush” process as an “unraveling” of a decentralized market, which also occurred in other areas like the market for medical students entering residency programs.

To combat this unraveling, the National Panhellenic Council adopted a centralized system of matching new members to sororities, the Preferential Bidding System.

According to Aaron Roth, assistant professor of Computer and Information Sciences, implementing this kind of matching algorithm means that there’s no incentive to recruit for a sorority or a fraternity early because the person will be guaranteed to be matched.

At Penn, the system is used throughout formal recruitment to narrow down people’s preferences and to eventually offer them bids.

According to an email from College junior Rachel Ruda, vice president of recruitment for the Panhellenic Council, the potential new members and either the chapter presidents of the recruitment chairs enter their list of preferences into a central computer program called Campus Director, one of the two programs used for sorority recruitment.

This program matches girls’ preferences with the sororities’ preferences, narrowing down the list of houses they visit throughout recruitment and ultimately determining the best outcomes for offering bids.

One of the main differences between these two algorithms, Alvin Roth said, is that NRMP guarantees “stable matches” while PBS in sorority recruitment does not. “If some doctor isn’t matched with some hospital, it should be because they filled the position with people they liked better,” he said. In other words, NRMP would never create a match in which both the applicant and the hospital would rather be matched with someone else.

However, in the case of sorority bids, not all matches are necessarily stable. Sororities impose quotas on the number of new members to whom they will offer bids in order to evenly allocate the girls across all of the sororities, according to Alvin Roth.

“What you want is to find the best outcome where the [new members] get matched to the sorority they most want and vice versa,” Aaron Roth said.

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