Archive for the 'Commons seat predictions' Category

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Marginal improvements. Looking at the reliability of seat predictions from polls

Monday, February 5th, 2018

Last week, Mike Smithson noted the Conservatives seem to have an in-built advantage in the electoral system over Labour – if they got an equal number of votes, the Conservatives could expect about 15 seats more than Labour even if Labour had a 0.5% lead in the polls, if Electoral Calculus is to be believed. That begs the question whether seats are likely to move consistently at the next election in the way that seat predictors assume. Let’s have a look at the possibilities.

Electoral Calculus uses what is called a strong transition model. For major parties in seats in which they are in contention, this is close to uniform national swing. For present purposes, I’m going to stick with looking at uniform national swing, which has the great merit of simplicity.

First things first, what is uniform national swing? It assumes that as votes move from one party to another, they will do so in precisely the same percentages in every constituency. So if the Conservatives hold a seat with 51% of the vote and Labour have 46% of the vote, a 5% swing would see those percentages reversed. If UNS applies, then if the Conservatives had just 20% in a seat and Labour had 70%, we should expect the Conservatives to drop to 15% and Labour to get 75%. The point to note is that you look at the absolute percentages, you don’t try to adjust for the proportion of voters in each constituency.

Logically, UNS can’t predict seat counts in all contingencies – it breaks down completely mathematically in more extreme cases (this is what Electoral Calculus’s adjustments for a strong transition model are designed to overcome). Obviously, seats do vary according to local circumstances and the quality of candidates on offer, and few seats will swing by exactly the national swing (Mike’s home seat, Bedford, is quite unusual in that it has tracked the national swing closely at every election since its creation in its modern form in 1997). In practice, UNS has done pretty well in normal circumstances in coming up with broadbrush predictions, particularly in two party systems.

Why? Two different points are worth considering. First, when you’re dealing with a large number of swing seats, you’re likely to produce a bell curve of distributions of swing, clustered particularly around the median swing. All other things being equal if the swing isn’t outlandishly big, those large numbers of seats are likely to be almost randomly distributed by size of majority across the bell curve. A seat might fall to an unexpectedly big swing but it is likely to be counterbalanced by a seat that didn’t fall owing to an unexpectedly small swing.

Secondly, seats of a similar type are likely to behave similarly (and probably did so in the past too, assuming they haven’t changed). Lincoln, Bolton West and Keighley, for example, have oscillated between the two main parties for decades. So the ordering of seats at a previous election is likely to be quite indicative of the ordering of seats at the next election, though the colour of the rosette of the winner might change.

Do these considerations hold next time? Well, the second one looks very suspect. Here are the marginals that the Conservatives are defending and here are the marginals that Labour are defending (including all seats vulnerable to a 10% swing). Focus on the final column, which shows you what the swing was last year. Nationally, the swing was about 2.2% from the Conservatives to Labour.

But when you look at the most marginal seats, the swings vary wildly and no pattern is easy to discern. Stroud and Bishop Auckland nestle side by side in the Labour defence list at numbers 12 and 13, but Stroud secured an outsized swing to Labour while Bishop Auckland saw a solid swing to the Conservatives. On the Conservative defence list at numbers 12 and 13, Broxtowe and Stoke-on-Trent South show similar and similarly contrasting swings.

The last election saw relatively few seats change hands but both main parties’ coalitions seem to have changed substantially, whether because of Brexit or because of the very different direction that Jeremy Corbyn has led Labour. It seems unlikely that seats that responded very differently to those stimuli will respond similarly to either the continuation or the withdrawal of those stimuli. Broxtowe and Stroud might swing in similar ways next time, and Bishop Auckland and Stoke-on-Trent South might swing in similar ways next time, but all four swinging in harmony does not look particularly likely.

There is a betting consideration to this. Next time the constituency markets will not be proxies for the national result. You won’t be able to deduce from a 3% swing in the polls from the Conservatives to Labour that Chingford & Woodford Green, North East Derbyshire and Carlisle all look in more or less the same amount of peril for the blue team. Constituencies will need to be considered either individually or in relatively small groups.

But what of the bell curve? I’ve had a look at the 2017 results and plotted the seats by size of swing, grouped into bands, here. As you can see, this looks pretty much like a bell curve centred around the 2.2% swing to Labour I mentioned earlier. Not only that, the seats in each segment divide roughly evenly between the blue and the red team in each band (with the exception of those seats which swung up to 5% to the Conservatives, which are twice as likely to be Conservative-held as Labour-held).

So UNS, and similar methods, look as though they still have life in them. What you shouldn’t do next time is look at a given seat and extrapolate even indicatively how it is likely to perform next time from an opinion poll. Local considerations look set to be far more important than national trends when looking at individual seats.

Alastair Meeks




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Latest Electoral Calculus projection has CON 15 seats ahead even on a 0.5% lower average vote share

Thursday, February 1st, 2018

This is, of course, on the current old boundaries

I have made this point before but the latest projection from Martin Baxter’s Electoral Calculus is an excellent example of how under current boundaries the system works in favour of the Conservatives.

As can be seen in the January projection above LAB had a 0.5% average poll lead but when fed into the Martin Baxter computation Corbyn’s party ends up with 15 fewer MPs.

The boundary changes, if agreed, make this even more pronounced. If GE17 had taken place under the proposed new electoral map the Tories on June 8th would have secured a majority of 4.

Bedford, where I live, is one of the changes, moving, according to Baxter from a LAB majority of 789 to a CON one of just 9 votes. making it just about the tightest marginal in the country. It is one of just two seats where the gap is down to single figures.

All this means that LAB has to be securing polling leads which are considerably bigger than that which they have been enjoying recently something that doesn’t seem to dampen the red team’s rhetoric.

Mike Smithson




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Seat projection from today’s ICM poll has CON ahead on MPs even though behind on votes

Tuesday, January 16th, 2018

When’s the Corbyn Clique going to work out that the system now works against LAB?

The latest ICM Guardian poll out and the figures – C40/LB41/LD7 – are included above in the seat projection from Martin Baxter’s Electoral Calculus.

As can be seen that although the Conservatives are one point behind on votes this, according to the projection, will put them one ahead in terms of seats.

This reflects a big trend that first was noticed at GE2015 when the Lib Dems were hammered after the years in Coalition. The Tories who had suffered most in relation to LAB on the votes/seats in the previous four elections found themselves benefiting disproportionately from the sharp decline of what was then Clegg’s party.

    The effect is that now on the same vote share the Tories will probably get 20-30 more seats than Labour. If the system is biased then it is to the blue team though they’ll continue, no doubt, to whinge quoting 2005.

LAB, for want of a better term, “wastes” more of it votes chalking up big shares in its heartlands than the Tories who are more vulnerable to the LDs.

Interesting in the latest projection above that the LDs move up two seats even though their share is down on GE2017. The reason, of course, the that in this poll the Tories are down even more.

Projections on the proposed new boundaries have the system biased even more to the blue team.

All this means is that Mr. Corbyn’s LAB needs vote share leads far in excess of anything it has managed to achieve since June to be sure of him becoming PM.

Mike Smithson




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In late April the Tory data chief, Jim Messina, told senior Tories that his modelling pointed to a CON majority of 290

Tuesday, December 5th, 2017

Put this on your “Christmas” list

I’m just back in the UK after my holiday on the West Coast of the US visiting my son, Robert, and his family who have moved to LA from London in July.

Part of my holiday reading was Betting The House: The Inside Story of the 2017 Election by Tim Ross and Tom McTague which was published last month.

It is an absorbing read giving a detailed account of GE2017 based on conversations with many of the key players and provides interesting revelations that look remarkable given what we know now.

The top point for me that put all this into context is the one highlighted in the heading – what Tory chiefs were being told ten days into the campaign. This is an extract.

“.. Ten days into the campaign, Jim Messina, the American data consultant working with the Tories, told Stephen Gilbert, Lynton Crosby, Mark Textor and other senior figures that his modelling suggested the Conservatives would win 470 seats – enough for a staggering majority of 290, more than double Margaret Thatcher’s 1983 landslide – and an exponential improvement on David Cameron’s winning margin of twelve.

It was an extraordinary moment and one that caused serious concern among those at the top of the campaign who already feared expectations were spiralling out of control. Messina’s forecast was the high-water mark for the tide of Tory optimism, but right up to election day the most senior campaign officials thought they would make strong advances into Labour territory… “

Although this inevitably got ratcheted down as the campaign progressed the view throughout the seven weeks that an increased majority was a certainty had a totally adverse impact on Conservative thinking. Quite simply it skewed the party’s whole management of the election and approach to seat targeting.

This is how the authors describe what happened when in late May a poll had the lead narrowing sharply. They were so convinced of the outcome that it was dismissed.

“.. On May 25th YouGov ran a poll in the Times, cutting the Tory lead over Labour to just five points. It was the first clear sign that a real change could be happening but was widely dismissed by commentators and analysts as unrealistic. Five days later, YouGov produced something even more dramatic: a seat projection model that said the Tories were on course to lose their majority in a hung parliament. Jim Messina and Mark Textor did not believe it. Sitting inside CCHQ, Messina composed a message on Twitter: ‘Spent the day laughing at yet another stupid poll from .@yougov. Hey .@benleet do you want to bet for charity? I’ll take the over.’ Messina showed it to colleagues and asked if he could tweet it, before doing so…”

But it wasn’t just the Tory campaign that was getting a distorted view of what was happening. Labour’s private pollsters were also giving a gloomy picture for their client. This from just before polling day:

“.. The picture from Labour’s own pollsters BMG was pessimistic. For most of the campaign, BMG had been forecasting a Tory majority of 150. On election day, they thought May was on course for a majority of 80…”

In a telling point on LAB targeting the authors report that activists believe they would have come a lot closer to the CON seat totals if they’d known what was really happening.

If you are being pressed to suggest ideas of Christmas presents for yourself then mention this book. It is a must read for all who follow polls and election forecasting.

Mike Smithson




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June 8th 2017 is a day that the election predictor/modellers will want to forget

Wednesday, June 14th, 2017


Wikipedia

It is little comfort to the election predictors/modellers that Wikipedia has now decided to record for posterity how successful they were in predicting the party seat outcome of GE17. The chart is above.

As can be seen only the YouGov model based on 50k+ of its own interviews came out of this well.

Throughout the campaign the forecaster/modellers aimed to produce projections of the party seat totals which, are course, based on the outcome of 650 separate first past the post elections in the different constituencies. This is sharp contrast to the BREXIT referendum or, say, the final round of the French election where it is a binary choice of two based on national totals.

The reason the GE17 modellers got it so terribly wrong was that their main data sources, the opinion polls, had, with one very honourable exception, a huge polling fail. Never was the saying garbage in garbage out so appropriate.

Virtually all of the models were following standard swing theory in their approach to seat predictions which meant a big error on top of everything with their LD seat projections. The party increased its MP total by 50% with a reduced national vote share. The exception was YouGov with its own exclusive and large polling data source.

A problem was the overwhelming CON landslide narrative which dominated everything and was reinforced by the adjustments almost all the pollsters had made following the GE15 polling fail. Any pollster that produced numbers that didn’t fit the general perception were attacked and their findings ignored.

This meant that sharp move away from the Tories after the manifesto saga was much less noticeable at the time and things like the consequential drop in CON 65+ support are only now being observed.

Lots of lessons from GE17 then which no doubt they will try to avoid next time.

The problem, of course, is that there is a huge appetite during a campaign for information on how the battle is going in terms of seats. Gamblers in particular are a key audience. The forecaster/modellers satisfy that need.

No doubt the Wikipedia table will be rolled out next time as a reminder to treat projections with caution.

Mike Smithson




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The CON GE2015 target seat over-spending issue throws into question the mathematics of GE2017

Monday, May 1st, 2017

Keeping within the limits could make a difference

Becasuse so much of the effort to predict and analyse the next election is based on what happened in each seat at GE2016 we are in something of a quandary because of what we know now about the Conservative approach to constituency expense limits.

It is entirely possible that the Tories will be defending seats on June 8th that would not have been won if spending had been kept with the constituency limits.

Before GE2015 political scientists such as Prof John Curtice were predicting that the Conservatives would need a GB vote margin over Labour of 11% in order to achieve a majority. That they reached that target with a gap of just 6% was a remarkable feature of the election and how many punters got it so wrong.

Now thanks to the work of Channel 4’s Michael Crick and others we have a greater awareness of what went on in the election and this has been closely studied by the Electoral Commission which was very critical of the party.

    Thus we cannot now say with certainty that seat X requires a swing of Y% because the base figure, the GE2015 result, could have been different if spending limits had been kept to

What would have happened in that seat if expense limits had been adhered to and that is very much in the air.

This is all going to make betting on some single constituencies much more difficult and also raises questions the standard poll shares to seats calculations

If we do get news of action by the CPS before polling day that could also have an impact.

Mike Smithson




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How Scotland and the LD collapse almost completely reverse the bias in the electoral system

Friday, May 29th, 2015

The dramatic shift in Britain’s political landscape

As we all know one of the constants in British politics over more than a quarter of a century has been that the electoral system has been “biased” towards Labour. Essentially for a given vote share the red team will have more MPs than the blue one.

Well the big news from May 7th is that that is all over and now the Tories will get more seats for an equal vote share than Labour. This is largely because of the total LAB collapse in Scotland and the Lib Dem decline.

The details are set out in illuminating article by Tim Smith of the University of Nottingham just published. He writes:-

“The largest contributor to this shift was third party victories, which swung from a Labour lead of 21 seats to a Conservative lead of 39 seats. The pro-Labour element of this had been mainly due to the fact that there had been far more Liberal Democrat MPs in seats where the Conservatives would otherwise have won than in those where Labour would otherwise have won. The collapse of the Liberal Democrats to just 8 seats eliminated most of this. Meanwhile, the SNP landslide in Scotland then pushed the bias in the other direction making Labour the primary victim of third party wins…

..In the UK system the boundaries are not deliberately gerrymandered by partisan redistributions, but nevertheless, they now very much favour the Conservatives whose votes are much more efficiently distributed. When the parties’ vote shares are equalized, Conservative wins waste far fewer surplus votes than Labour, with the latter now tending to pile up larger but ultimately unnecessary majorities in safe seats. The reason for this big increase in Conservative efficiency was caused by their very strong performance in the right places, i.e. marginal seats, and this was helped by the large number of first term incumbents standing for re-election for the first time. Labour did best in its safest English seats.”

This means, of course, that new boundaries would make the system even more favourable to the Conservatives.

Mike Smithson





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The Thanet S & Hallam polls fail to move the markets & CON a 75% chance to win most seats

Thursday, April 30th, 2015

This is a bet on seat distribution not national vote shares

One of the big things to remember as we get close is that the final seat totals are not governed by national vote share in some apparently pure manner but on the specific outcomes in 650 separate constituency battles fought under FPTP.

Clearly this time the traditional ways of estimating seats from polls numbers have been smashed to smithereens by the political earthquake in Scotland. We should treat the politics north of the Tweed like we do with Northern Ireland which has operated in its own political eco-system for half a century.

In England and Wales where the big LAB-CON battles will take place it is not inconceivable that the party that’s second on votes could come on top.

So much is determined by local activity and strength of the party’s organisations.

When I look at the Ashcroft seat polls where the outcome is tight I always check the voting numbers before turnout filtering comes in. Quite often, like in yesterday’s Ashcroft Swindon South poll the Tories were 2 points adrift before this calculation.

As I’ve observed many times in the key marginals the party machines’ objectives are to ensure that even the most marginal voters vote.

My reading of the most seats outcome is that CON leads but not by very much. We are 55-45

Mike Smithson

For 11 years viewing politics from OUTSIDE the Westminster bubble