Inspired by the example I posted recently, I suggest to discuss a new independence criterion. It is in the same spirit as IPDA and ISDA in that it requires the winner to stay the same when an alternative is added which is very weak in a sense. More precisely, the new alternative is considered weak when the new defeats it brings are too weak to be considered essential new information. Here's the definition:
INDEPENDENCE OF WEAKLY DEFEATING ALTERNATIVES (IWDA): The winner must not change when an alternative X is added but for each option Y which is defeated by X, already some stronger defeat against Y is affirmed/undropped by the method. In the current wording, IWDA only applies to methods which affirm defeats (like Ranked Pairs, River, Kemeny) or drop defeats (like PC, SD, SSD). Ranked Pairs and Kemeny fail IWDA, as can be seen from the example I posted recently. In that example, C is the "weakly defeating alternative". Does SSD (Beatpath) fulfil IWDA? And how does IWDA relate to IPDA and ISDA? Jobst ---- Election-methods mailing list - see http://electorama.com/em for list info
