What can Westminster results tell us about Assembly results? 2. Plaid Cymru

How well did Plaid Cymru’s performance in 2015 predict the party’s performance in 2016? Like Labour, there is a very strong relationship between their share of the vote in the two elections. The correlation between the two, for the statistically minded reader, is .91. Again, constituencies above the line of best fit are where performance was relatively better in 2015 and constituencies below the line are where performance was relatively better in 2016PC2015

Interestingly Plaid’s three Westminster seats (Arfon, Dwyfor Meirionydd, and Camarthen East and Dinefwr) are all above the line; perhaps this is explained by regression towards the mean. Seats where Plaid are strikingly stronger in the assembly include Gwent, Aberconwy, and Cardiff West.It’s worth mentioning that UKIP didn’t run in Arfon or Aberconwy. This, as I’ve discussed previously, would have likely had a greater impact on Labour’s vote share than Plaid’s, which would be an issue in Arfon where Labour are Plaid’s rival. In Aberconwy, held by the Conservatives, the evidence is less clear cut that UKIP’s absence would have benefited Plaid – Conservative vote share was higher where UKIP’s was too.

In terms of change in vote share, there is a stronger relationship (the correlation is .44) than seen with Labour, but it’s certainly much weaker than for overall vote share. Interestingly Plaid’s disappointing Llanelli result was arguably better than might be expected based on 2015, while Leanne Wood’s triumphant win in The Rhonda might have been expected to be even bigger – unusual verdicts certainly. Clwyd South, not a target seat for Plaid, stands out as somewhere where their direction of travel was better in 2015, as does Arfon.


Finally, as can be inferred by the difference in the number of constituencies Plaid hold in the Assembly (6) compared to Westminster (3), Plaid are much more successful in the assembly than they are at Westminster. They get a mean of 21% of the vote in assembly constituencies as opposed to 12.9% in the Westminster equivalents. This isn’t due to a sudden surge in Plaid support though, their mean gain in vote share was 1.4% in 2016, compared to 1.2%.

Next time, the Conservatives….


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