[EM] Question for STV-PR supporters

James Gilmour jgilmour at globalnet.co.uk
Wed Jul 16 17:10:10 PDT 2003

Alex asked:
>  Or, what is the typical
> district size in places that use STV-PR for public elections?

I provided some answers to that question last night.  So now to the more
general questions.

> How many members should a typical legislative district elect?
> So what district size would you recommend?

There is no "typical" (or "correct") number of members that a
legislative district should elect.  It is all a matter of trade-offs
among factors that pull in opposite directions.  The greater the number
of members elected together, the more proportional the result will be.
But proportionality is not the only factor in representation in the real
world.  It is certainly not the only factor of concern to real electors.

If a town council has 12 members, it is quite practical and acceptable
to elect all 12 by STV-PR from a single, town-wide district (US
terminology), as the examples from Ireland show.  But if we are looking
at a national parliament of, say, 120 members, it  would be undesirable
to elect all 120 from a single national-district.  The principal reason
is not one of practicality, but rather political consequence.
Experience has shown that it is politically undesirable to give
representation down to 1 in 120, as the example of the Israel Knesset
shows.  In countries using party list systems of PR with national
aggregation of votes to determine the parties' shares of seats, it is
common to apply an arbitrary threshold (typically 5%) specifically to
exclude small parties.  These thresholds are arbitrary (Why 5%?  Why not
4% or 6%?); it would be more logical to accept the de facto thresholds
that come with districts - where the sizes of the districts have been
determined for other, logical reasons.

For this reason I would reject all "extensions" of STV-PR that seek to
add any aggregation of votes across districts with the aim of
"improving" the PR.  Such aggregation is undesirable, flies in the face
of the local candidate-based nature of STV-PR, and as experience has
shown, is unnecessary to obtain an acceptable result.

The law of diminishing returns applies to representation in much the
same way as it applies to many other things.  As the number of members
per district rises, the proportion of voters NOT guaranteed
representation falls off very rapidly at first and then progressively
more slowly.  (This applies to all systems of PR, not just STV-PR.)  So
there is a trade-off between the proportion of voters not guaranteed
representation and the political consequences of giving representation
to ever smaller groups with ever decreasing support among the
electorate.  Of course there will be debate about where along the curve
of diminishing returns the district size should be set.

One special problem in addressing this question for STV-PR is that many
analysts look only at the number of seats a party will be guaranteed for
a specified proportion of the first preference votes.  They ignore
completely the effects of vote transfers.  They argue for large district
magnitudes to ensure fair representation of the smaller parties.  But
small parties that cannot muster, say, one-sixth of the first preference
votes in a 5-member district frequently win one seat in such districts.
There are many examples of this in the results from the STV-PR elections
in the Republic of Ireland (since 1920) and in Northern Ireland (since

There is also a trade-off between the proportionality of representation
and the localness of representation.  These two factors pull strongly in
opposite directions so far as optimal district size is concerned.  In
some political cultures, real electors attach great importance to
localness and so it cannot not be ignored.  This is certainly true in
the UK and in New Zealand. It is much less important in most of
continental Europe.  I would guess it could be an important factor in
the USA and in Canada, but I have no first hand experience on which to
base a firm opinion.

Proponents of so-called "pure PR" may say that the electors have got
their priorities wrong, but practical reformers would be unwise to
ignore the electors' desire to secure discrete representation for the
"natural communities" that exist within the boundaries of legislatures
at all levels.  It is to give expression to these natural communities
that it is also common to vary the district magnitude among the
districts within a legislature.  PR purists may protest that this
introduces urban/rural bias, and party bias where the support for
parties varies from district to district.  But experience shows that
real electors are more concerned about their communities than about the
exactitude of proportional representation.

Thus the legislation for local Councils in Northern Ireland stated that
districts should elect not fewer than four members and not more than
eight, and that five, six or seven should be regarded as the desirable
number of members per district.  In the STV-PR elections to the Scottish
Education Authorities in the 1920s there was even greater diversity,
from 3 members to 10 members, and there was great variation within
individual Authority areas, e.g. Fife with seven electoral districts of
six different sizes: 9, 8, 8, 7, 5, 4, 3.

Pragmatic flexibility in practice.

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