That could be one big poster where the candidates are listed on the
right hand side and the left hand side is used for representing the tree
structure (and the names of the parties and the subgroups).
That could work, at least in cases where there's only one district and
the party limits the depth of the tree so it doesn't get too cluttered.
I don't think it would be of much use in small-district elections, since
either all subgroups would have to field candidates (making the regional
lists very long), or only some subgroups would have candidates you could
vote on.
To be absolutely safe, each party in party list PR would have to have at
least as many candidates in the running for a region as there are seats
in the region. If you want absolute representation not just between
parties but within the party, each subgroup would have to do the same,
which would add greatly to the count.
It could be solvable by something like MMP where you have one
constituency (FPTP or STV) vote and one subgroup vote, where the
subgroup totals propagate up the party -- or for an asset-flavored
method, the subgroups negotiate with weighted votes as to how the list
votes are to be divided up.
Asset-flavored methods would have the same Fiji-type problems you
referred to, however, if the candidates throw their weight behind
something you don't support. One might say that feedback would make the
voter trust the candidate less the next time around and thus keep them
in line, but that argument could be made for Fiji, too, and observation
shows that feedback isn't strong enough.
The substituted ranks (candidate-individual automatic how-to-vote
cards) would nest outwards, from the small wings to the increasingly
larger ones within the party itself, then on to other parties in
preference. In a sense, they are "lists" of their own, and so the
problem isn't completely avoided.
Are there some specific cases where the tree like inheritance order is
clearly not sufficient?
Not really - it was more of an addon to increase the information given,
with the idea that if the candidates transfer beyond their own party,
then a socialist green could favor other socialist and green parties
over conservative ones, for instance. Since I considered the
tree-structure too complex, I thought that complexity wouldn't be an issue.
At least the rank substitution method gives a simple way of implementing
such a nested party-list method. If candidates declare subgroups, and
subgroups supergroups, then the rank votes are generated so that a list
gives the ordering within subgroups, and then the ranked vote is
[Candidates in subgroup] > [ Candidates in other subgroups ] > [
Candidates in other supergroups ] and so on.
That brings us to the original question, whether it'd be possible to
simulate this method. If "tree-based party list" (as we may call it) is
transformed to STV, then any question of proportionality of tree-based
party list would be reduced to a subset of the questions of
proportionality of STV alone, and so it wouldn't be necessary to test
tree-based party list separately - at least not unless the structure
mitigates some problem with ordinary STV representation.
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