[EM] Runoff vs IRV

[MIKE OSSIPOFF]

Though it would be hoped that we'd get something better than Runoff or IRV, 
what if it were necessary to choose between those 2?

Of course they both have advantages compared to eachother, and examples in 
which they do better than the other.

But what stands out, for me, is that Runoff always elects a CW who comes in 
1st or 2nd in the initial Plurality count, whereas IRV can fail to elect a 
CW who is favorite to more people than anyone else is.

I've posted examples of that happening. I'll post them again here. They 
aren't contrived or implausible examples. All it takes is for favoriteness 
to taper away from the voter median position. That's a very plausible 
scenario.

First I'll write this example in full, then I'll simplify it by leaving out 
the preferences that IRV never looks at.

Example 1, complete:

67: ABCDE
73: BACDE
100: CDBEA
84: DECBA
70: EDCBA

Example 1, simplified:

67: AB
73: B
100: C
84: D
70: ED

It looks a bit sparse, doesn't it, after we delete the preferences that IRV 
disregards.

When A & E get eliminated and transfer inward, C becomes the candidate with 
fewest votes, though C is favorite to the most people. And of course C is 
the CW too.

Example 2, simplified as before:

49: AB
52: B
100: C
54: D
48: ED

This is an extreme example to show how badly IRV can fail in that way with 5 
candidates.  Example 1 is more typical. In Example 2, not only is C the CW, 
and not only is C favorite to more voters than anyone else is, as in Example 
1, but this time C is favorite to about twice as many people as anyone else 
is.

Now, as you said, IRV can save a CW who's Pliurality score is only 3rd 
largest. But sometimes it won't. But the IRVers emphasize the importance of 
favoriteness--that's their whole argument for IRV against Condorcet. So 
then, how do they justify letting a big-favorite candidate (and CW) lose, 
giving up Runoff's reliable protection for 1st or 2nd-placing CWs, in order 
to maybe, sometimes, protect a 3rd place candidate?

It doesn't sound like much of a trade.

It isn't as if IRV could guarantee that a CW would win if s/he were 1st, 
2nd, or 3rd in favoriteness, while Runoff only guarantees that s/he wins if 
s/he is 1st or 2nd. IRV makes no such guarantee. As shown above, IRV can 
fail to elect a CW who is favorite to more people than any other candidate 
is--even one who is favorite to about twice as many people as any other 
candidate is.

In terms of concrete guarantees for electing a popular CW,  Runoff looks 
better than IRV.

Isn't a 1st-placing or 2nd-placing CW more likely than a 3rd placing 
CW--which IRV doesn't reliably protect anyway?

The Runoff proposal for state elections would be a big improvement over 
Plurality, and seems to me to be better than IRV too. The Runoff proposal 
should be supported unless we can get another, better, proposal ready in 
time to compete with it. Or maybe just find out what happens this time, and 
we can offer Approval or Condorcet next time.

Mike Ossipoff

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