Adopting IRV will cause voter errors ("spoiled" ballots) to become
7 times more frequent
(based on San Francisco numbers).
But adopting score voting appears to decrease errors.
Score voting is monotonic, i.e. increasing your vote for somebody can help
but cannot hurt them.
IRV is not monotonic.
In score voting, scoring your favorite candidate top cannot hurt either
you or him.
In IRV, it can hurt you.
With IRV the "Nader spoiler" and "wasted vote" problems are
contrary to pro-IRV-propaganda.
Indeed, because of fear of these very effects
(or for whatever reason), IRV voters tend to rank third-party
candidates below top (even if favorite) and hence prevent their election,
which presumably is why every
IRV country is and always has become 2-party-dominated.
For this reason IRV cannot attract support from intelligent third-party members.
IRV makes ties and other nightmare-scenarios much more likely;
Score voting makes them much less likely.
IRV is historically more likely than score voting merely to lead to
a backslide to plurality voting.
IRV will (in plausible scenarios)
elect candidate X in preference to candidate Y, even though
based on the IRV ballots themselves,
Y is pairwise-preferred over X (and over everybody else too)
by an arbitrarily-huge supermajority of the voters.
This happened in
the Burlington 2009 mayoral election
and appears to have happened in
both the Peru 2006 presidential election
(but less dramatically; merely a "55% majority" rather than a "huge supermajority" was
thwarted) and the Chile 1970 presidential election
(this time with about a 2:1 ratio supermajority being thwarted).
Probably this fiasco had something to do with why Burlington repealed IRV.
See also our reports on the
IRV elections and their pathologies.
These were some of the most important IRV
Raising your most-hated candidate in your IRV vote from bottom to top-ranked
can actually improve the
election winner (in your view)!
IRV must be counted centrally and cannot
be counted in precincts, there is no longer
such a thing as a precinct "subtotal." This can decrease election transparency and
raise the risk of a centrally organized fraud.
Score voting is less complicated than IRV, both for the vote-counters, and also for the voter in
the sense that experimental timings show voters score candidates faster than they rank-order them.
indicates score voting also is more popular than IRV with real voters.
Indeed, the most experienced IRV-using country by far, Australia, wishes it could
get rid of IRV so much, that they'd actually go back to plain plurality voting
(by large margins in 3 polls) if offered that choice in a referendum; also
IRV was rejected by massive 68-32 margin
in favor of plain plurality in a 2011 nationwide referendum in the UK;
meanwhile poll evidence from France indicates voters would, if given the choice in a referendum,
switch to score voting right now (and they prefer it over approval voting too).
IRV exhibits an artifical bias favoring electing extremist
candidates and against centrists.
In contrast approval can have pro-centrist bias, and with
score voting there
is little or no bias favoring either.