Placeholder Image

字幕列表 影片播放

  • It is the hot topic for data journalists in this election.

  • Some call it MRP, some Mr P. But the full name

  • is multi-level regression with post-stratification.

  • So what is it?

  • In short, it's a way of using a big national poll

  • to estimate how people will vote at constituency level.

  • National polls of 1,000 people are

  • good at telling us what share of the national vote

  • each party will get, but not so good at predicting who

  • will win each of the 650 seats.

  • And in the UK system it's seats, not votes, that counts.

  • For example, in 2015, the Conservatives

  • won 37 per cent of votes and 51 per cent of seats.

  • Ukip won 13 per cent of the vote and less than 1

  • per cent of seats.

  • Traditionally, pollsters have tried

  • to get around this using a method

  • like uniform national swing.

  • If Labour was down 11 percentage points on the last general

  • election nationally, the pollsters would subtract 11

  • percentage points from their vote share in every seat.

  • But that can't capture all the electoral nuance, for example,

  • the influence of Leave and Remain in particular areas,

  • or big student votes in university towns.

  • So in comes MRP.

  • Step one, a large poll sample, tens of thousands

  • of people across the country.

  • That's because you want dozens of people in each constituency,

  • the more the better, to pick up on what

  • makes that seat different from the rest.

  • Step two.

  • Don't just ask them who they're voting for.

  • But who they are.

  • Age, sex, ethnicity, education level,

  • housing, occupation, how they voted in the EU referendum.

  • You'll also have gathered lots of local information

  • about their constituency, from which parties have historically

  • done well or poorly there, to what's

  • happened to house prices.

  • So you have data at the individual level,

  • but also the context of the wider geographical area.

  • That's why it's called multi-level.

  • You then run a regression on that data.

  • That's a statistical technique that

  • measures the probability of someone

  • with those combinations of personal and local

  • characteristics, A, voting at all, and B,

  • voting for a particular party.

  • So we've done MR. Then comes P, post-stratification.

  • This is where the modellers use data

  • from sources like the census and the annual population survey.

  • They can tally up the number of people

  • with each combination of these demographic and socio-economic

  • characteristics in every constituency

  • and then apply the voting probabilities from the MR step

  • onto the population data.

  • So you have an estimate for how a white British male who

  • left school at 16 is likely to vote, for example.

  • And that estimate will differ between Great Grimsby,

  • Northwest Durham, and Glasgow Central.

  • In fact, you have a series of estimates

  • for different demographic combinations

  • in different places.

  • So you then combine them to give you

  • the total number of votes each party is likely to secure

  • in every one of the 650 constituencies,

  • and that can be used to calculate which party

  • is most likely to win each seat, which is most likely to be

  • its closest challenger, how big the margin

  • between first and second place is likely to be, and so on.

  • This allows parties to better target their campaigning

  • resources on seats that are going to be close.

  • And it can help people like you make a more informed decision

  • on who to vote for tactically.

  • It gives the public a more nuanced picture

  • of how the election is likely to play out.

  • Now, it's not a perfect system.

  • This type of modelling is complex,

  • and there are many variables.

  • And the choices the modellers make mean one model

  • will have different outputs to the next.

  • But MRP is the most refined tool we have

  • until the votes are actually counted.

It is the hot topic for data journalists in this election.

字幕與單字

影片操作 你可以在這邊進行「影片」的調整,以及「字幕」的顯示

B1 中級

民調機構如何預測英國大選結果? | 報道 (How do pollsters predict UK general election results? | FT)

  • 0 0
    林宜悉 發佈於 2021 年 01 月 14 日
影片單字