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# Voting matters - Issue 10, May 1998

## Producing a Party List using STV

#### C Rosenstiel

Colin Rosenstiel is a member of the Council of ERS and the author of a
computer program for STV.

With some of the current proposals for electoral reform, parties will be
required to produce a list from whom candidates will be elected in order
from the top. STV can be used to construct the ordered list, given a
preferential ballot of all party members.

The conventional use of STV to elect *n* members gives members of equal
status, since the order in which STV elects does not necessarily determine
the strength of their support. Repeated use of STV elections can be used to
determine an order as follows:

Given a total list of 10 (say), then the first step is to determine those on
the list (without an ordering) by running an STV election with all the
candidates and 10 seats to fill. The next step is to run an STV election for
9 seats with 10 candidates being those previously elected (using the same
ballot papers). The eliminated candidate is then placed last on the list.
Next, an STV election if run with the remaining 9 candidates with 8 seats to
determine the next lowest candidate, and so on.

This process might sound tedious, since so many STV elections are run, but
if a computer is used, it is straightforward. Note that the above process
will not work in reverse, i.e. selecting the top candidate first. The reason
for this is that when electing two candidates, it can happen that neither of
those elected is the previously selected 'top' candidate.

Two elections were taken in which there was more than ten
candidates to which we have applied the algorithm above to
order the top 10 candidates. The results obtained was as
follows:

Election 1 Election 2
This algorithm : ABCDEFGHIJ ABCDEFGHIJ
Order of election : CABDEFGHJI CBAFEHDGIJ

As expected, it can be seen that the order of election does not give the
same result as successive elimination. Hence this algorithm is recommended
in producing party lists.
### Editorial comment

It has been suggested to me that if the Meek method is used, then just one
election would suffice (to determine the order of the 10 candidates). Their
order can be found from the retention factor in the final table of the
election results - the smallest retention factor implying the
strongest candidate since that candidate required the smallest proportion of
the votes retained to get the quota. These values do give a measure of their
relative support, unlike the order of election. In the elections above the
Meek result were:
Election 1 Election 2
Meek 'keep' factor: ABCDEFGJHI ABCEFDHGIJ

This would appear to indicate that the methods of ordering of the candidates
produce a similar result. In practice, both methods would need to use a
computer and hence there seems to be little to choose between them.

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