The representativity of a random sample: the need for mandatory participation

The unexpected Conservative majority in the 2015 UK general election has led to considerable agonising in the polling industry. Why were the polls — which consistently predicted a hung parliament, or even a Labour victory — so wrong? A polling industry enquiry has come to some interim conclusions, here paraphrased by the BBC political editor, Laura Kuennsberg:

Pollsters didn’t ask enough of the right people how they planned to vote. Proportionately they asked too many likely Labour voters, and not enough likely Conservatives.

Nobody is suggesting that this bias was intentional — it was the accidental by-product of the polling methodology which, rather than drawing a random sample and then knocking on doors (the gold standard, but very expensive), relied heavily on telephone and online surveys. The problem with phone surveys, according to Martin Boon from ICM, is that out of 30,000 random numbers only around generally 2,000 agree to participate. This sample is anything but representative:

Adam Drummond, from Opinium, suggested that the people who agreed to be interviewed were so politically enthusiastic that the would even vote in elections for the European Parliament. . . This seems like a Groucho Marx problem — people answering the phone and agreeing to answer questions about their political opinions automatically means that they are too politically engaged to be representative.

Online polling is even worse:

Online polls are answered by a database of volunteers who have signed up to be on a panel and who the company knows a lot about. When the company is commissioned to do a poll it can be sure that the people it is asking have the same features as the whole population in — for example, the proportion of men and women, or the age profile or income distribution.

However what stratified sampling does not do is to control for the level of political engagement, so

The online polls came out with much the same inaccuracies as the phone ones. Does people’s willingness to be on a panel automatically make them unrepresentative?

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The difficulty of insuring accurate randomness

This article points to the problem of how to produce true representation in juries.

Dearbhail McDonald: The verdict is in – our jury selection process is a farce

Juries are meant to be representative of society as a whole. However, they are anything but, writes Dearbhail McDonald

Trial by jury is one of the last remaining sacred cows in the criminal justice system.

Born by accident to replace trial by ordeal and duelling, amongst other dispute resolution techniques, the random selection of 12 peers is still prized as the only anchor by which a government can be held to the principles of its constitution.

Lord Devlin, the celebrated British judge whose father was from Co Tyrone, famously described jury trial as the lamp that shows that freedom lives.

Here’s the salient point:

For the most part, trial by a jury of one’s peers is as unquestioned as it is innate.

But our current system of selecting juries makes a mockery of jury trial as a bulwark against State power and other anomalies.

To fulfil their constitutional mandate, juries (which only featured women from as late as 1976) are meant to be representative and jurors drawn from a complete cross-section of the community.

They are anything but.

In practice, the burden of jury duty is disproportionately borne by Dubliners; the young, the old and retired, the unemployed, civil servants or those who can manage to undertake the difficult task. The recent empanelling of a 15-strong jury … brought home to me the challenges of achieving the “constitutional completeness” of the representative jury.

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