Political

How accurate are the Thrasher and Rallings local election predictions for the Conservatives?

I’ve previously talked about the accuracy, or not, of the Thrasher and Rallings prediction for the Lib Dem vote share in local elections, and for Labour, but what about the Conservatives?

Here is how the Thrasher and Rallings predictions have panned out for Conservative vote share* based on by-election results in advance of the big May round of elections.

 

Forecast Tory % Actual Tory % Error in forecast
2006 33% 39% -6%
2007 39% 40% -1%
2008 n/a 43%
2009 40% 35% 5%
2010 n/a 39%
2011 35% 38% -3%
2012 34% 33% 1%
2013 29% 26% 3%
2014 30% 30% 0%
2015 n/a 36%
2016 31% 32% -1%
2017 35% 39% -4%
2018 36% 37% -1%

Unlike the consistent over-estimation of the Lib Dems, for the Tories these predictions have a lot more variation. On average the Tories slightly over-perform the forecasts, but the level of variation makes that a pattern of limited use for interpreting future forecasts.

To see the full context of the predictions and results each year, see my PollBase collection of local election results.

* Different rounds of seats are up for election each year, so Thrasher and Rallings calculate a national equivalent vote, which adjusts for this and therefore make possible like-for-like comparisons between different years. The intent is the same as for the separate BBC / Curtice and Fisher calculations, though following a different methodology and hence also usually producing slightly different results. The fact that however they both, with their own different approaches to the issue, consistently produce the same trends and overall picture – e.g. this time Lib Dems moving back above Ukip – is a good sign that the pictures they paint are right.

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