[EM] Burlington Vermont repeals IRV 52% to 48%

robert bristow-johnson rbj at audioimagination.com
Wed Mar 3 12:25:54 PST 2010

On Mar 3, 2010, at 2:53 PM, Bob Richard wrote:

> Kristofer Munsterhjelm wrote:
>> [snip]
>> What method will be used in Burlington now -- Plurality or runoff?  
>> Since you said 40% earlier, I guess it's a runoff, but 40% sounds  
>> odd as a runoff threshold. Shouldn't it be majority? Anything less  
>> and the voters might have preferred someone else.
> The argument for 40% as opposed to 50% comes from political  
> scientists and is practical rather than conceptual. Essentially it  
> is that a plurality winner who gets 40% is extremely unlikely to  
> lose a runoff against the second-place candidate, so that the  
> runoff isn't worth the additional expense (to the candidates and  
> voters as well as the government). A refinement on this is the  
> "double complement rule", described here:

> http://fruitsandvotes.com/?p=546

i'm having a little trouble decoding some of this (quoting):

There are better ways to determine when a plurality is sufficient and  
when there should be a top-two runoff. For instance, the double  
complement rule, first proposed in 1994 by Rein Taagepera and yours  
truly (in Comparative Political Studies). Under the DCR, in any  
election in which no candidate obtains over 50%+1, there is a runoff  
if (and only if) the second candidate’s shortfall from majority is  
less than double that of the leader.

In other words, if the leader has 44%, he is six percentage points  
short of 50%. There would be a runoff if the second-place candidate  
had more than 38%, which is double the leader’s shortfall from  
majority. If the second candidate is under 38%, the election is over  
in one round, with the leader’s 44% sufficing. Obviously, the gap  
required between the top two candidates to avoid a runoff shrinks as  
the leader approaches 50% and increases as the leading candidate’s  
plurality decreases–as it should. So with a leading candidate at 40%,  
there would be a runoff unless the second candidate had less than 30%.

The DCR is not actually used anywhere, but it was the inspiration  
behind the rule adopted in 1994 in Argentina when that country junked  
its US-style electoral college. The Argentine rule is a bit more  
complex. A leading candidate with 45% wins in one round under any  
circumstances, even if the runner-up is at 44.99%. And less than 40%  
for the first candidate necessitates a runoff no matter how far the  
runner-up trails. But in between 40% and 45%, the first round is  
decisive only if the leading candidate has a ten-percentage-point  
lead over the runner-up.


how does that get us to this artificial 40% threshold?

BTW, in deep conceptual physics (like "Theories of Everything" where  
descriptions of interaction try to combine gravitation to the other  
forces), i like to think of using "Natural units" or "Planck units",  
so we don't get any of these unnatural, anthropocentric measures and  
thresholds, because of our choice of units.  that's a little OT, but  
i find it hard to see the number "0.40" as anything other than a  
number that came outa somebody's butt.  if a plurality winner gets  
40%, then no opponent can have as much, but could be close.  electing  
a candidate with 40% + 1 vote when his opponent got 40% + 0 and there  
is virtually 20% of the vote up in the air, just cannot be considered  
a safe determination of the will of the people.


r b-j                  rbj at audioimagination.com

"Imagination is more important than knowledge."

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