Does Clive Lewis’s Progressive Alliance add up electorally?

Yesterday Clive Lewis suggested that Labour should form a progressive alliance with the Greens, SNP, Plaid Cymru and the Lib Dems, even going so far as to say “frankly, I want to be in government with Caroline Lucas, not against her.”

This certainly rattled some cages in many parts of the Labour Party. Setting aside the arguments around the desirability of this proposal, it set me wondering if such an alliance could actually win the next general election. So I took the model I produced last August, to see if non-voters could help Labour win the next election and modified it.

The modified model assumes some form of electoral pact in England where Labour do not stand candidates where the Greens and the Lib Dems have MPs. Likewise the Greens and Lib Dems do not stand candidates in any of the other seats Labour are fighting.

The model then allows you state what percentage of Green and Lib Dem voters will vote Labour if the party they voted for in 2015 does not stand a candidate. I’ve assumed 90 per cent of Green voters will vote Labour and 66 per cent of Lib Dem voters will vote Labour (it seems likely to me a smaller proportion of Lib Dems than Greens will switch to Labour). But you can change the figures in the interactive model below.

At the end of this post are more details about the model, but you can see that such a progressive alliance would be hard to form, even accounting for a Corbyn-led Labour Party enthusing some non-voters to vote Labour. Of course, there are other factors to consider in Clive’s proposals and the model is a simple one, but it does give an idea of the numbers.

And now, on to the model. See below. Enter your values in the red boxes and the graph will change. Note the graph gives the seat for each party and the bar at the bottom gives the total seats of the “progressive alliance” of Labour, Lib Dems, Greens, SNP and Plaid Cymru.

As ever if you think I’ve got anything wrong please let me know in the comments field.

If you want to use it on a phone or tablet you need to double tap one of the red cells to get it to bring up the keyboard.

  1. The turnout in the 2015 general election was 66.1 per cent.
  2. You can find the general election turnouts since 1945 here and you’ll see that turnout was only above 80 per cent twice: 83.9 per cent in 1950 and 82.6 in 1951. Originally my model has assumed a Corbyn-led Labour Party could get turnout up to 75 per cent, but a few people pointed out at nearly a 10 point increase this was unrealistic. So I’ve changed it to 72 per cent as this matches the turnout in the EU referendum. But you can change this parameter yourself on the model.
  3. It would be wrong to assume all the new voters because of increased turnout would vote Labour, some would be motivated to vote Tory to “stop Corbyn.” Originally I assumed 85 per cent of new voters would vote Labour. Again many people said this was unrealistic as the evidence is non voters tend to split on similar lines to the other party votes. Fair enough but it is clear Corbyn is engaging with the disaffected so it is plausible to assume the non voters who do turn out are more likely to be Labour voters, and I’ve set this at 66 per cent. You can change this parameter yourself on the model.
  4. My model only looks at seats in England and Wales as and assumes SNP will retain all their seats but will form part of a progressive alliance.
  5. Finally, this is based on the 2015 boundaries and the Tories intend to reduce the number of seats from 650 to 600 and they will seek to get boundaries redrawn in such a way that it benefits them. So whatever the results this model shows, it will be harder for Labour if the boundaries are redrawn.

Dodgy use of opinion polls

This graphic has been doing the rounds on Twitter. It is perhaps one of the most misleading use of opinion poll data I’ve ever seen. 

The graphic comes from this Ipsos MORI poll. Here is the link to the charts for the poll and I’ve reproduced the page with the graph on below. 

You can see there are actually two graphs. The one on the left is the one at the start of this blog post and is the one doing the rounds on Twitter. It shows Labour with a 5 point lead. The one on the right is shows a 1 point lead for the Conservatives. 

So what’s the difference? The one on the left gives the voting intention for all the people sampled. The one on the right gives “headline voting intention” which weights the result to take account of the likelihood of people to actually vote. 

This is massively important because if you don’t do this you get a distorted picture. Indeed in the 2015 general election the pollsters overestimated the Labour vote share. 

In their post election analysis the pollsters came to the conclusion that the reason they overestimated the Labour vote share was because they overweighted the likelihood of many Labour voters (particularly younger voters) to vote. 

So by choosing the graph on the left the person who tweeted this has presented a distorted picture that magnifies the errors in the polls done prior to the 2015 general election. No one credible ever quotes opinion poll results without weighting for turnout. 

They have either been guilty of blatant and deliberate cherry picking, or if I’m being generous, an unwitting confirmation bias. Either way it massively distorts the data. 

For the avoidance of doubt this post is not meant to be pro or anti any party, or party leader. It is meant to be pro-clarity and anti-distortion. 

And finally, I will add my six laws of opinion polls to help understand opinion polls more clearly. 

First law: If an opinion poll gives a result you don’t like and your defence is “they can’t be trusted” that’s fair enough.

Second law: If you have ever said “they can’t be trusted” but you have used an opinion poll to back up your preferred issue then you have broken the first law.

Third law: If the poll says what you would like it to say how sure are you that you are not guilty of confirmation bias?

Fourth law: Whilst voting intention polls are not great at precise predictions of vote share, they are broadly right. 

Fifth law: Remember every poll has a margin of error. Find out what it is.

Sixth law: It is dangerous to rely on one poll as the result could be an outlier within the margin of error (see fifth law) and/or you could be guilty of confirmation bias (see third law).