Wednesday, August 01, 2007

Saving lives with exact algorithms

It's not often you get to say this in a paper:
We aim for exact algorithms [because] ... any loss of optimality could lead to unnecessary patient deaths.
Anyone who's gone through an algorithms class will at some point hear about stable marriage algorithms, and how the method is used to match hospitals and newly minted MDs looking for residencies.

Consider now the far more serious problem of matching kidney transplant candidates to potential donors. Because transplant lists are long, and cadaver donors are few, marketplaces matching healthy donors to recipients have sprung up in the US. For complicated ethical reasons (which are not without controversy), such exchanges are not made for money, and are viewed as gifts.

So what happens if a donor kidney doesn't match the potential recipient ? Ordinarily, nothing. Suppose however that there was another donor-recipient pair with a similar mismatch, and if only the donors were swapped, both transplants could go through ? What about if a 3-way cycle of matchings could be found ? This is called a 'market clearing', and is the subject of a paper by Abraham, Blum and Sandholm to appear in EC.

I'll get into the problem statement in a second. What's far more important is that the results of this paper have already been used to find potential transplant matches where none existed ! The Alliance For Pair Donation has been using this method since last December for finding potential matches, and has already found matches that prior methods would have missed. This is an incredible achievement: working on a problem abstracted from a real life-or-death scenario, and actually taking the results back to the source and making a difference.

Technically, the problem is easily stated. Given a directed graph with weights on edges, and a parameter L, find a maximum weight collection of disjoint cycles, each of length at most L. Vertices are agents with items (in this case, transplant recipients with donors). The weight on an edge represents the utility to the source of obtaining the sink's item (a donor transfer). The L-constraint reflects reality, in that all such transplants would have to be performed simultaneously (to ensure that all donors go through), and it's not feasible to perform more than a few (typically 3) of these transplants simultaneously.

The line I quoted at the beginning of the post comes as part of an explanation as to why they want exact algorithms for the problem (it's NP-hard for L >= 3). The technical contributions include finding a way to run an integer-linear program at scale for graphs with hundreds of thousands of nodes.

(Via NSF Current)
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