Election outcome – what?!

The news outlets have now spent the last 16 hours finding as many superlatives as they possibly can to describe the election we just witnessed. As Britain went to the polls, the opinion pollsters continually had Labour and the Tories neck and neck:

polls-pre-election

The picture doesn’t make it totally clear (it’s all polls with a smoothed line plotted through), but a visit to, say, UK Polling Report’s polling average makes it more clear.

The outcome, however, is that the Tories polled 36.9% of the vote nationally, and Labour 30.5%, and subsequently the Tories have managed to win a majority, with 330 seats as I write, one more to declare.

The discussion has at least in part centred on why the polls were so wrong. I want to add a minor quip to all of this. As part of my forecasting course I prepared a simple linear regression based forecast model that simply uses nationwide polls alone to predict seat outcomes. I presented it to my students back in March (slide 10), and then also quickly referred to it in a talk at Nottingham Business School on Wednesday (slide 6). Here’s the forecast(s):

model_forecast_2015_080515

Marked on are the outcomes, as they stand. The regression model took each opinion poll with its projected vote share, and also corrected for the number of days until the poll, and whether a party was incumbent. It combined these variables using interaction terms, but remained nonetheless a simple linear regression model. Nothing special.

But it does get Labour’s seat total pretty much bang on. A couple of opinion polls were as optimistic as what’s occurred, but the majority aren’t all that far short of what the Tories ended up with, and certainly this model did predict a much wider gulf between the parties than the naked polls alone did.

What does this say? It probably says that if we corrected polls for their historical performance in predicting seat outcomes, they’re not that far away from what actually happened, in reality. This method does also bias correct polls as well, should they display any bias towards one party or another, and adds a control for an incumbent party – which raises their seat totals, and hence the Tory total being much bigger than Labour despite polling neck and neck.

I’m sure nonetheless that pollsters will get it in the neck, but I thought I’d just point this out…