[EM] Holding byelections with PR-STV

[Raph Frank]

One issue with PR-STV is that there is no clear way to handle byelection.

In a 5 seat constituency, a party with 17% of the vote is entitled to a
seat.  However, if that person was to die or resign the seat, then the party
is not likely to win the byelection, as the larger parties have a big
advantage.

One option is just to have a custom that byelections are not contested.
However, that removes from the public the ability to express their opinion
in byelection.

The issue with holding a full new PR-STV election is that sitting
legislators might end up losing their seats.  This seems unfair (and
certainly would seem unfair to legislators).

The solution in Australia is the count back procedure.  This requires
storing the ballots for all constituencies.  The ballots are recounted as if
there was a new election.  However, the person who resigned is eliminated
and all sitting legislators are protected from elimination.  The final seat
is then filled proportionally.

In Ireland, the seats are filled via PR-STV with one seat, so effectively it
is IRV.  I think that if 2 TDs from the same constituency were to vacate
their seats near each other then a 2 seat election would be held.

Also, there is a philosophical argument.  The effect of the count back
procedure is that people who have died/left the constituency since the last
election get their vote counted, while new adults/people who have move into
the constituency don't get their counted.

OTOH, a byelection (in general) gives 1 constituency a chance to bargain
with the government.  If the government has a weak majority, they may
(unfairly) be able to get concessions.  Also, everyone is in the same
situation, each constituency is represented base on the most recent
election.

Anyway, assume the goals are that a new election is held, but that sitting
legislators are guaranteed a seat.

If they are directly protected from elimination, then there is no point in
their supporters voting for them, so a complete guarantee wouldn't work.
Voters would just vote for non-sitting candidates and it is back to IRV.

They could be given some partial protection against elimination.  For
example, they could be to give them a boost in votes when considering
elimination.

Each sitting candidate's vote total could be considered to be
(Votes+Quota)/2, when deciding who to eliminate.  This should mean that they
can easily defeat members of their own party who aren't sitting candidates
(which tends to be where the main competition comes from anyway).

However, if nobody votes for a sitting candidate, then that candidate will
lose.  Thus, their supporters would have an incentive to vote for their
favourite candidate, even if he had a seat.

Also, since elimination order doesn't affect proportionality, it would still
be proportional.

Another option would be to have a rule that no more than 1 sitting candidate
can be eliminated.  Once a sitting candidate is eliminated, then the others
are protected from elimination.  That could possibly lead to tactical
voting, but is probably more trouble than it is worth (and it is unlikely
that any candidate would be willing to be "sacrificed").
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