Under the Fixed Term Parliaments Act, it’s possible that Westminster elections will regularly precede elections to the assembly by a year. Given how little polling is conducted for Assembly elections (with the honourable exception of the collaboration between Prof. Scully at Cardiff University,  ITV Wales, and YouGov), this could be a really helpful indicator of the likely outcome of the Assembly elections. But how well did results in 2015 predict assembly results this May? I’ll start with Labour and work my way through the other parties in subsequent posts.

I plotted Westminster vote share (Y-axis) against Assembly vote (X-axis). The colour of text reflects the winner of the assembly seat. There’s clearly a strong relationship here, indeed the correlation is .897, which is very high by the standards of the social sciences. Constituencies under the line of best fit are where Labour did better in the assembly than you might imagine based on Westminster results, those above the line are where they did worse. Gwent and the Rhondda stand out as you’d imagine, but most constituencies are very close to the line, as you’d imagine with such a high correlation.

This is probably unsurprising. You would imagine that places inclined to vote for Labour in Westminster would be fairly likely to vote for them in the assembly too. What about change in vote share? Do shifts from 2010-2015 also predict shifts from 2011-2016? I plotted change in vote share in the two elections against each other in the same way

Here the relationship is much weaker, with a modest correlation of .286. Interestingly the outliers are somewhat different. Gwent was still a surprisingly big loss in Labour vote, but so was Swansea West, where Labour were very much on the front foot in 2015. The defeat in the Rhondda, interestingly was actually quite predictable given the result in 2015 and, based on its position below the line of best fit, could have been much worse for Labour.

Finally, correlations ignore the means of each set of votes. It’s worth pointing out that Labour did much worse in the Assembly elections than in Westminster. Their average vote share fell from 40% to 38.6%. More strikingly, Labour’s mean vote change was a gain of .6% in 2015 but a mean loss of 7.8% in 2016.

Next on Cyfri am Byth, Plaid Cymru…

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