How big a lead does Labour need in the local elections to be on course to win the next general election?

By Stephen Fisher.

People often wonder what local election results bode for the next general election. Here is a quick look at the relationship between the BBC Projected National Share of the vote (PNS) in the local elections and the outcome of subsequent general elections. In particular, I focus on the lead for the main opposition party over the principal governing party. The graphs below show the lead at the subsequent general election plotted against the lead in the PNS.

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Calculating the local elections Projected National Share (PNS) in 2018

By John Curtice and Stephen Fisher.

Part of the ritual of election-night coverage at the BBC is the calculation of the so-called Projected National Share (PNS). This is an estimate of the share of the vote that the principal parties would have won in a GB-wide general election if voters across the country as a whole had behaved in the same way as those who actually voted in the local elections. It provides a single, seemingly straightforward measure of party performance that can tell us not only how well or badly a party has done as compared with four years ago, but also as compared with any previous local elections for which a PNS is available – even though the places in which local elections are held varies considerably from one year to the next.

One of the key patterns in last year’s general election results was a tendency for those who voted Remain to swing more to Labour than those who voted Leave, while the Conservatives lost ground amongst Remain voters while advancing amongst their Leave counterparts. If a similar pattern is maintained at these local elections – and it was in last year’s county council elections – then the Labour vote will increase more (or fall less) were the Remain vote was higher in 2016, while the converse will be true of the Conservatives.

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Forecasting Local Election Seat Gains/Losses 2018

by Stephen Fisher

My forecasting model for seat gains/losses at local elections has previously been a simple model based on change in party support in the polls. While the historical data show that changes in the percentage of the council seats that a party wins is reasonably strongly correlated with changes in that party’s poll share, that basis for forecasting this year would not work.

The Conservatives are up by 8 points and Labour up by 6 points in the polls since just before the 2014 local elections, when most the seats up for election this week were last fought. As in last year’s general election, support for both main parties has increased largely because of a big drop in UKIP support. Forecasting the parties separately based on changes in their respective poll shares would produce misleading predictions of big gains for both main parties. In first-past-the-post elections it is typically relative not absolute performance that matters for seat outcomes. Last year, despite winning more votes the Conservatives lost their parliamentary majority because the Labour vote share went up by more than did that for the Conservatives.

For this reason, my forecasting models this year are based on changes in the gaps between polls shares.  For the Conservatives, who have traditionally faced many contests with the Liberal Democrats, their leads over both Labour and the Liberal Democrats matter. For Labour, the model is primarily based on the Labour lead over the Conservatives. Meanwhile, for the Liberal Democrats, their changing opinion poll performance relative to the Conservatives, but not Labour, has historically been correlated with headline local election seat changes.

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