[EM] Approval Voting vs Instant Runoff Voting

[LAYTON Craig]

Mike wrote:

>In Approval, people will vote only for their favorite if they think
>they don't need their 2nd choice, because their favorite has a win.
>Actually many who now voted only for Nader, because that was the only
>way to express support for him, would vote for Nader & Gore in Approval.
>So fewer, not more, people would vote only for their favorite.

Hold on.  You only recently finished telling us that Nader could win, if
only you had a different electoral system.  So, which is it?

>by voting someone else over him. IRVies say you can safely rank your
>favorite 1st. They forget to mention that that's only true if your
>favorite is a sure loser.

To a certain extent, it's also true of a favourite & compromise vote in
Approval.  It only works if your favourite is a probable loser.  If your
favourite is neck and neck with a less favoured, but not the worst, popular
candidate, it's almost impossible to work out how to vote.  The consequence
is that everyone uses their vote in different ways, and the result is
largely random.  See my example of opinion poll ratings & expected utility
outcomes and try to decide how to cast your Approval vote.

>In IRV, a CW can be favorite of more people than any other candidate is,
>and still lose. That's a very plausible IRV scenario, and is likely

As demonstrated, this can also happen in Approval.  The fact that it is
somewhat less likely isn't particularly comforting.

>Not reassuring when your crucial 2nd choice compromise isn't there
>anymore because he got eliminated while your single traveling vote
>was on your favorite. Your vote will not be dropped? Your preference
>for Middle over Worst won't get counted. At least Approval reliably
>counts every preference that you vote.

This is only true because you're forced in such a situation by Approval to
only express one preference - either favourite over middle or middle over
worst.  IRV counts one of these preferences, Approval counts one of these
preferences.  I don't see how you can say one is worse than the other in
this regard.