Extensions of partial priorities and stability in school choice

We consider a school choice matching model where priorities for schools are represented by binary relations that may not be linear orders. Even in that case, it is necessary to construct linear orders from the original priority relations to execute several mechanisms. We focus on the (linear order) extensions of the priority relations, because a matching that is stable for an extension profile is also stable for the profile of priority relations. We show that if the priority relations are partial orders, then for each stable matching for the original profile of priority relations, an extension profile for which it is also stable exists. Furthermore, if there are multiple stable matchings that are ranked by Pareto dominance, then there is an extension for which all these matchings are stable. We apply the result to a version of efficiency adjusted deferred acceptance mechanisms.