Stepwise ordinal efficiency for the random assignment problem

We introduce a notion of efficiency, called Stepwise Ordinal Efficiency (SOE), and prove that it coincides with a fairness notion of interim favoring ranks, in the sense of Harless (2018). We also prove that SOE implies ordinal efficiency, while it is not compatible with rank efficiency. Then, we provide an impossibility result which states that no mechanism meets SOE, weak strategy-proof, and strong equal treatment of equals. Finally, we show that a modified eating algorithm satisfies SOE.