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During the current batch of moderator elections at Gaming, it has been argued that since only 2 seats are up for grabs in this round of elections, it only makes sense to cast ballots by only ranking our top 2 choices. After all, in the sites' first round of elections, we were asked to rank the top 3 candidates and 3 was also the number of seats up for grabs.

I always thought the "rank the top 3 candidates only" thing was put in place to simplify the voting process, as the precision gain of ranking more candidates (and thus picking a 4th choice, and a 5th choice, and a 6th choice...) would have been too little to make a difference.

In this sense — the tradeoff between the precision of the election process and the ease of voting — does it make sense to rank $k > n$ candidates in a Meek STV election when only $n$ winners are going to be declared?

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If I was looking for examples where "rank top $k$ candidates only" would not suffice, I'd consider a scenario where each voter strongly detests one candidate, but ranks all others more or less equally. It at least seems intuitive that not being able to rank all candidates in such a situation would strongly reduce the ability of voters to accurately express their preference. –  Ilmari Karonen Aug 18 '11 at 13:03
    
@Ilmari Yes, but at least in the context of Stack Exchange, there's a (optional) round of filtering via the so-called primary phase that effectively removes candidates people "detest" by asking them to vote candidates up and down, then taking the top few candidates by score. –  badp Aug 18 '11 at 13:07
    
Good point @badp. My answer does not attempt to include the rest of the Arqade scenario. –  Mark Hurd Aug 18 '11 at 13:16

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I don't know the actual maths, but there are definitely examples in Australian elections (which is only similar) where relatively low rankings have made a difference to the result.

Note also it depends upon the number of candidates: if you have 20 candidates for 3 positions, it is quite possible 10 to 15 rankings could still not provide a voter with a real choice in the result.

As @IlmariKaronen suggests, if you really dislike some candidates you want to be able to put them as low as possible in your rankings. (The alternative approach of not ranking them is not the best approach if you really dislike the candidate.)

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