Tag Archive for 'FPTP'

Both before and since the UK election in May, there has been a lot of talk of electoral reform and how it would affect British politics and the makeup of Parliament. As the Liberal Democrats have secured a referendum on bringing in Alternative Vote, I thought it would be a worthwhile exercise to develop a full simulation of how elections under Alternative Vote would work out in the UK, based on the results from May 6th.

While there are full methodological details below, the model I’ve developed is a full and thorough simulation of AV, modeling every round of transfers in all 650 constituencies, based on educated estimates of the transfer rates between different parties. I’ve run the model twice, under slightly different assumptions. In the first version of the model (which I’ll refer to as the simple model), I have assumed that the votes cast on the 6th of May would be the first preferences of voters in an AV election. This is a nice simple assumption to make, but it doesn’t take into account that people vote tactically in first-past-the-post (FPTP), and often won’t actually vote for their preferred candidate if they don’t think he has a chance of winning. AV would remove the incentive to do so, so you would expect that first preferences would be different than votes cast under FPTP.

In an attempt to account for this, I’ve also run a second version of the model (which I’ll call the adjusted model). One of the notable features of the recent election is how different the actual result was from what people were saying to pollsters right up to the day before the vote. It’s a reasonable hypothesis to say that the polls were reporting the actual preferences of voters, and that the shift on election day was the result of people voting tactically due to FPTP. As such, for the adjusted model, I’ve applied a proportionate adjustment to the first preference votes in each constituency, to make the national tallies add up to what the pre-election polls were predicting. It’s hoped that this adjusted model can better capture the true first preferences of voters than the simple one.

Anyway, on to the results. After running through the model for each set of assumptions, here are the predicted seat totals for the simple AV model (AV-Smp), the adjusted AV model (AV-Adj) and the actual election results for reference (FPTP):

What’s immediately obvious is that, as would be expected, the Lib Dems gain the most benefit from the new system, as they’re the party most disadvantaged by FPTP. What might be surprising, though, is quite how much they benefit from it when you look at the adjusted model. It’s often pointed out that AV is not a truly proportional electoral system, which is quite true. However, my model suggests that it would be far more proportional than the current system, with the Lib Dems winning 23% of the seats on 27% of the vote, rather than the current 9% of the seats on 24% of the vote.

The model also suggests that coalition government could well become the norm under AV, as under both simulations a government would need any two of the big three parties to form a majority. Nonetheless, this simulation is being applied to an outlier election in the first place, and it’s still entirely possible for parties to gain an overall majority under alternative vote.

It’s worth pointing out that this isn’t developed as a predictive model of elections under AV; it’s a simulation of what might have been on May 6th, not what will happen in future elections. I might amend it in future to analyze likely results in the next election (if it’s held under AV), but the output will be unlikely to closely reflect what you see above, due to changes in political dynamics between elections. The Lib Dems, for example, are likely to lose out on transfers from Labour and independents after going into government with the Tories, but will be more transfer-friendly to Tory voters.

Continue reading for full methodological details >>