[EM] MOP-F2 / Weak Defensive Strategy criterion, Strong Defensive Strategy criterion

[Juho Laatu]

> On 08 Jul 2019, at 22:05, C.Benham <cbenham at adam.com.au> wrote:
> 
> 
>> Non-Drastic Defense refers to performance with strategic votes.
> http://mam-docs.000webhostapp.com/ <http://mam-docs.000webhostapp.com/>
>> non-drastic defense <http://mam-docs.000webhostapp.com/Proof%20MAM%20satisfies%20Minimal%20Defense%20and%20Truncation%20Resistance.htm>:  If more than half of the voters prefer alternative y 
>>         over alternative x, then that majority must have some way of voting 
>>         that ensures x will not be elected and does not require any of them to 
>>         rank y over any more-preferred alternatives. (This has been promoted 
>>         by Mike Ossipoff under the name Weak Defensive Strategy Criterion.  
>>         Non-satisfaction means some members of the majority may need to 
>>         misrepresent their preferences by voting a compromise alternative 
>>         over favored alternatives if they want to ensure the defeat of less-
>>         preferred alternatives.) 

The electowiki definitions of Weak Defensive Strategy criterion and Strong Defensive Strategy criterion seem to play funny word games. Strange things are hidden in the supporting definitions, e.g. on what voting two candidates equal is supposed to mean. Truncation pops up again as a point with a special meaning. Some detective work needed to find out what is allowed and what not.

Let's study an alternative, clearer and a bit stronger defensive strategy criterion.

"If a majority prefers one particular candidate to another, then they should have a way of voting that will ensure that the other cannot win, without any member of that majority changing their rankings."

In MOP-F2 this means that voters in that majority should leave their rankings intact and only (possibly) change the position of the cutoff. In a three candidate election where C is the candidate that the majority doesn't like, those majority members that prefer both A and B to C should put their cutoff after A and B (this means A>B>>C or B>A>>C with fully ranked votes). Other majority members should not use the cutoff. I couldn't identify any scenario in MOP-F2 where C could still win, when majority members follow this strategy.
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