Assaf Romm

Strategic Behavior in a Strategy-proof Environment

A mechanism is said to be strategy-proof if no agent has an incentive to misrepresent her true preferences. This property is considered highly desirable for mechanisms that are used in real-life markets. And indeed, many of the great success stories of market design employ strategy-proof mechanisms, such as the second-price sealed-bid auction (Vickrey 1961), or Deferred Acceptance (DA, Gale and Shapley 1962; Dubins and Freedman 1981; Roth 1982). Specifically, in school-choice settings, the appeal of strategy-proof mechanisms is one of the main reasons many school districts choose the applicant-proposing version of DA over pre-existing mechanisms. At the core of the attractiveness of these mechanisms is the assumption that agents report their preferences truthfully in strategy-proof environments. We present direct field evidence of preference misrepresentation by applicants under the applicant-proposing DA. Our study is based on the recently redesigned admission process in Israel for graduate studies in psychology (MA or direct PhD track), where several participating institutions offer positions in the same study track, but under different terms. In particular, in some cases only some of the positions are funded.

Redesigning the Israeli Medical Internship Match

The final step in getting an Israeli MD is performing a year-long internship in one of the hospitals in Israel. Internships are decided upon by a lottery, which is known as the Internship Lottery. In 2014, we redesigned the lottery, replacing it with a more efficient one. This article presents the market, the redesign process, and the new mechanism that is now in use. In this article, we describe the redesign and focus on two-body problems that we faced in the new mechanism. Specifically, we show that decomposing stochastic assignment matrices to deterministic allocations is NP-hard in the presence of couples, and present a polynomial-time algorithm with the optimal worst case guarantee. We also study the performance of our algorithm on real-world and simulated data.

Assigning Israeli medical graduates to internships

BackgroundPhysicians in Israel are required to do an internship in an accredited hospital upon completion of the medical studies, and prior to receiving the medical license. For most students, the assignment is determined by a lottery, which takes into consideration the preferences of these students.ObjectivesWe propose a novel way to perform this lottery, in which (on average) a larger number of students gets one of their top choices. We report about implementing this method in the 2014 Internship Lottery in Israel.MethodsThe new method is based on calculating a tentative lottery, in which each student has some probability of getting to each hospital. Then a computer program “trades” between the students, where trade is performed only if it is beneficial to both sides. This trade creates surplus, which translates to more students getting one of their top choices.ResultsThe average student improved his place by 0.91 seats.ConclusionsThe new method can improve the welfare of medical graduates, by giving them more probability to get to one of their top choices. It can be applied in internship markets in other countries as well.

'Strategic' Behavior in a Strategy-Proof Environment

We present direct field evidence of preference misrepresentation under deferred acceptance. A large fraction of highly educated participants, who had been informed about the strategy-proof nature of the mechanism in numerous ways, failed to play truthfully: they ranked a non-funded position above a funded position in the same program. This is despite being assured that rank-order lists are never made public, that funding is a positive signal of ability, and that funding comes with no strings attached. Surveys indicate that other kinds of preference misrepresentation are also prevalent. Preference misrepresentation in the field is associated with weaker applicants. A laboratory experiment shows that the same individual is much more likely to misrepresent her preferences when she expects to be ranked low relative to others. Our findings have important implications for the study and design of matching marketplaces.

Implications of capacity reduction and entry in many-to-one stable matching

In two-sided many-to-one matching markets some mechanisms induce worse allocations for one side of the market following a capacity reduction on the other side. This prediction, however, is not true for all matching mechanisms. Assuming preferences are strict and responsive, we are able to provide comparative statistics involving subsets of agents on both sides of the market for all stable mechanisms. Within the larger domain of substitutable preferences capacity reductions may have ambiguous consequences. Nevertheless, if preferences satisfy the law of aggregate demand a similar result does hold. These results are an extension of the one-to-one results on entry of Roth and Sotomayor (Econometric Society Monographs No. 18, 1990) to many-to-one environments. Finally, we consider truncation strategies, and describe how agents may manipulate the matching process to their advantage, without knowing which stable mechanism is being used either by reporting a truncated preference or by shading capacity.

Solution to Exchanges 8.2 puzzle: a Dutch Dutch auction clock auction

This is a solution to the editors puzzle from issue 8.2 of SIGecom Exchanges. The puzzle is about finding a Bayesian equilibrium for a Dutch auction which can end according to a stochastic price schedule. The full puzzle [Conitzer 2009] can be found online at: http://www.sigecom.org/exchanges/volume_8/2/puzzle.pdf.