Predictions from Preferences

The way that the Alternative Voting system works is easy to describe but quite difficult to express in logical terms.  One of the trickiest aspects is deciding how the losing party's votes will be distributed to the other candidates.  I've seen some other posts where the 2010 election was "re-run" using AV system, but no detail was provided explaining the assumptions made.

So how should I do it?  Every voter is different, so voter A who voted for Labour might express Lib Dem as their second preference and Conservative as their third, whereas voter B might express Conservative as their second preference, and BNP as their third (a significant shift to the right, I would say!).

My suggested approach is to assume, for each party, their second preference parties only, and the mix.  When a party is eliminated under AV, these votes are transferred to the second preference parties in a pre-set distribution (another assumption needed for that), and from then on those votes "belong" to that party and inherit their second preferences.  In this way I only need to assume the second preferences and the mix.

It might not be too hard to make an educated guess for the second preference parties, but the mix will be a wild guess.  As an example, imagine the following votes in a seat:

Imagine the 100 UKIP voters express their second preferences as follows: 70 to Conservative, 30 to Labour.  The votes become after AV Round 1:

Finally, imagine Labour Party voters prefer Lib Dems to Conservatives in the ratio 70/30.  The 30 votes transferred from UKIP to Labour inherit this preference too, so the second and final round becomes:

So the Lib Dems win under AV, compared with the Conservatives under First Past The Post.



I shall finish the model and we'll see what the election might have been.