Nigel Marriott's Blog

An independent statistician using data to understand our world and to predict the future

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UK General Election 2017 #7 – Review of my predictions

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Exactly one month ago, the UK woke up to the news that they had elected a hung parliament for the second time in 3 elections.  For many forecasters including myself, this came as a surprise as I had been predicting a Conservative majority of 100 seats.  In the event, the largest ever polling underestimate of the Labour vote was enough to see the Conservatives lose their majority.

At the beginning of my commentary on election night itself, I defined success for my forecasts as being how close the number of Conservative seats was to my forecast of 375.  I also stated that if the number of seats was in the 340s I would consider this to be a prediction error.  The final outcome was 317 seats so clearly that is a major prediction error.

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UK General Elections #2 – How accurate are the opinion polls? – updated with GE17

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Ahead of the 2017 general election, I predicted that the opinion polls would be wrong again and that the Conservatives lead over Labour would be underestimated by 2.6%.   I based this on data provided by Mark Pack who has systematically recorded every opinion poll published since 1945.  In the event, I was right that the polls would be wrong but instead of an error favouring the Conservatives, the polls recorded the largest ever underestimate of the Labour vote.  As a result, election forecasters were blindsided yet again and the result was a hung parliament which few saw coming.

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UK General Election 2017 Results #1 – My Election Night Commentary

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Friday 07:35 – I’ve just woken up to the result that we have a hung parliament which is miles away from my prediction.  Clearly I need to conduct a post-mortem of my model but I did state in my first chart of this post (see bottom of post) that a hung parliament would occur if the CON-LAB lead was under 3%.  Sure enough that is the outcome with the BBC currently saying the CON-LAB lead is 2.3%.  What it also means that we have another epic polling error as the average of all polls in the week before the election showed a CON-LAB lead of 7.5%.  Unlike 2015 when all pollsters got it, congratulations must go to Survation who called it spot on.

I am sure we will have a wave of people claiming to have predicted this error.  As far as I am concerned a valid prediction is only one made in public in advance.  Such a prediction would also need to explain why they expected a polling error on the scale shown in the chart.

The last time such an error occurred was 1983 and the error is on the scale of 1951!  In fact the error is even more remarkable if you look at the 3 main parties.

Another polling post mortem beckons but this election will go down in history.

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UK General Election 2017 Forecast #4 – My Official Prediction as of 4th June 2017

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My official prediction using my Final Election Model is that the Conservatives will make a net gain of 45 seats resulting in a working majority of 105 seats.

My forecast uses data from my latest UK Opinion Poll Tracker and it is worth reading that post in conjunction with this post.  At the bottom of this post is a spreadsheet containing my prediction for each seat.  I am basing all figures in this forecast on the assumption that Conservatives will have a 9.5% lead over Labour on June 8th.  I arrive at that figure by taking the current CON-LAB lead of 7% in the latest polls and adding an expected 2.5% underestimate in the Conservative lead over Labour based on my analysis of historical polling errors.  A knock-on effect of this assumption is that I expect turnout to be 2pts higher at 68%.

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UK General Election 2017 Forecast #5 – 5 steps to making sense of the latest polls!

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Unlike the 2015 general election when the polls were essentially static (& wrong) throughout the election, the 2017 general election has seen some of the most extraordinary volatility in the polls that I can remember.  If you are a Conservative supporter, the narrowing lead over Labour must be leading to anxiety and changed underwear.  If you are a Labour supporter, you are probably starting to dream “can we? will we?!”  It doesn’t help that your state of mind will depend on which poll you are reading and your memories of the pollsters’ failure in 2015 so how can you make sense of what is going on.  I will show you how in 5 steps and to heighten the drama, I will leave the punchline to the end!

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UK 2017 General Election Forecast #3 – A Description of my Final Model

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For the last 6 weeks, I have been making forecasts of the number of seats that each party will get in the 2017 General Election.  If you have been following my forecasts, you will know that I have developed a variety of prediction models which all predict something different.  With 10 days to go, I decided it was high time to settle on a single Final Model which is described in this post.

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UK General Election 2017 Forecast #2 – Latest Prediction as of 21st May 2017

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Here you can find my latest forecast of the 2017 UK General Election on 8th June.  My forecast uses data from my latest UK Opinion Poll Tracker and it is worth reading that post in conjunction with this post.

My latest prediction using uniform regional swing adjusted for parties standing down is that the Conservatives will make a net gain of 52 seats resulting in a working majority of 119 seats.  If anti-Conservative tactical voting takes place then this has the potential to reduce the Conservatives working majority to 73 seats which I am sure Theresa May would still be very happy with! [Read more…]

UK General Election 2017 Forecast by Seat #1 – Bath, South West

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This post was first published on 29th April 2017 and predicted a narrow Conservative Hold.  I updated this post on 21st May 2017 to take into account latest data.

Welcome to the first of my constituency forecasts for the 2017 General Election.  I ‘ve chosen to start with the seat of Bath currently held by the Conservatives for two reasons.  First, it is where I live so I have a personal interest!  Second, it is the bell-weather seat for the Liberal Democrats when it comes to the success of the anti-Brexit strategy.  Fail to take Bath and they can kiss goodbye to any chance of making the election a success.

MY FORECAST – Lib Dem GAIN but still very marginal

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UK General Election 2017 Seat Forecast #6 – Edinburgh South, Scotland

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For my first Scottish seat forecast, I head up to Labour’s last seat in Scotland.   I have just become aware of a constituency poll in Edinburgh that took place at the beginning of April before the election was called but the data has only just been released by Survation.

At first sight the poll is bad news for the Conservatives and good news for Labour but I will show that in fact the reverse is the case and that the poll is further evidence that a Brexit Realignment may be taking place.  If so, the Conservatives majority in Parliament will be larger than current predictions are showing.

My Prediction – LAB Hold

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UK General Election 2017 Seat Forecast #5 – North Norfolk, East England

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This is my first seat forecast from the Eastern region and I have chosen the seat of North Norfolk for two reasons.  First it is one of only 8 seats held by the Lib Dems in 2015 and one of 3 Lib Dem-Leave seats.  Second, it is one of 23 seats in Britain this election that will be fought between only 3 candidates from the Conservatives, Labour and Liberal Democrats.  Both UKIP and the Greens have decided not to stand in this seat and this will be a good seat to examine my model for handling the votes of parties that stand down.

My Prediction – CON GAIN but extremely marginal

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