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In 2012, US statistician Nate Silver correctly predicted the results of all 50 states in the US presidential election with 100% accuracy.
Compare this with the shock result of the 2016 Trump election, and now the 2019 Australian election, when nearly all Australian opinion polls incorrectly predicted that Bill Shorten's Labor Party would defeat Scott Morrison's Coalition on a two-party preferred basis.
Pollsters - Result - Time
How did the pollsters get it so wrong? More importantly, how can we ensure we get a more accurate result next time?
A good opinion poll must do two difficult things:
it must get those voters to honestly divulge who they will vote for.
If both of these things happen, statistical theory gives us confidence that the poll will be reliable.
Difficulty - Opinion - Polls - Opinion - Polls
To add to the difficulty, we don't just need good opinion polls we need excellent opinion polls, because even a small error could mean predicting the wrong winner.
For example, if a government ministry wanted to know what percentage of people had done volunteer work in the last year, it probably wouldn't matter for their purposes if they commissioned two polls which gave estimates of 48% and 52%. But this variation is unacceptable for electoral opinion polls, which need a small margin of error to accurately predict the winner.
Representative - Sample - Voting - Population - Microcosm
A good representative sample will be the whole Australian voting population, but in microcosm. If there are 16 million eligible voters, then a representative sample of 1,600 would mean that each sampled voter represents 10,000 people just like them.
Using relatively small samples is a cheap way to estimate the bigger picture of the election, without resorting to expensive surveys like the Australian same-sex marriage survey.
Sample - Theory - Tells - Result - Margin
With a large and representative sample, statistical theory tells us that we will get close to the real result with a narrow margin of error. But the opinion polls in this election...
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