[EM] IRV vs Plurality Vote with a Runoff

barnes992001 at yahoo.com barnes992001 at yahoo.com
Fri Oct 26 10:04:44 PDT 2001

My student govt uses the following voting procedure, which I call the 
"Plurality Vote with a Runoff" (PVw/RO) procedure:

"The elected candidate shall receive 50% plus one or more of the votes 
cast. If more than two people are nominated, a run-off shall occur 
between the two people who receive the most votes in the first round of 
voting if in that round one person does not receive 50% plus one or 
more of the votes cast."

I am drafting a resolution to change that to Instant Runoff voting 
(IRV). It seems to me that the IRV and PVw/RO are mathematically 
equivalent with 3 candidates, given that the same voters vote and that 
they maintain the same sincere preferences from one balloting to the 
next. Furthermore, it also seems to me that IRV is either 
mathematically equivalent or better than PVw/RO according to all of the 
fairness criterion, including monotonicity and manipulability. I need 
to be sure about this, however.

        I know that they can yield different outcomes when there are 
more than 3 candidates. Is that also the case with 3 candidates?
        I know that neither is monotonic. As far as I can see, in a 3 
candidate contest, if one violates that criterion in a given example, 
then so does the other. Is that also the case with more than 3 
        I know the PVw/RO cannot elect the condorcet-loser. Can IRV 
elect the condorcet-loser?
        Does the fact that IRV uses a preference ballot, instead of a 
repeated ballot, make it less manipulatable?

Whatever you believe the answer is, can you prove it by example or 

Steve Barney

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