Tuesday, 15 September 2009

The Delphi coracle

I was determined not to jump into the explosion of blog comments on Derren Brown's lottery prediction trick - but something I heard yesterday has persuaded me to make the leap.

For non-UK readers, Derren Brown is an illusionist, who last week 'predicted' the lottery results on national TV - then a couple of days later, explained that he had got these results by having a group of people guess them, averaging the results, and reiterating the process. Allegedly, as the group got to know each other, the results got better.

I'm not bothered about the trick itself. Let's be clear - he didn't predict the lottery results, he showed them after they had already been broadcast, using a totally spurious legal argument as to why he couldn't show them before. So all the trick came down to was 'how did he transfer a set of numbers onto some balls quite quickly without us seeing?' - basic stage magic, nothing to do with seeing into the future.

What really irritated me was his 'explanation' show. He pointed out that if you get a group of people, ask them each what a cow weighs, then get them together and do the exercise again with interaction between the group, they come up with a more accurate figure for the weight of the cow. So far so good. This is the Delphi technique, devised by RAND for the US government. It works - it's a good way of getting to an answer with limited data. But, and here's the point, it's totally useless with no data at all.

If you asked the same group as did the cow's weight what a ptlang weighs, they will come up with a totally random set of numbers, and no amount of working together will get them nearer the right answer. Because they have no idea what a ptlang is, or what it might weigh. They do know what a cow is and, order of magnitude, that it's going to weigh more than a person and less than a car.

Just like the ptlang, they have no idea what numbers are going to be on the lottery balls that week, and no amount of working together will bring them closer. The 'explanation' was totally spurious - and, worse, could easily fool people into thinking this a realistic mechanism for predicting the future of random events, giving them a system for beating the odds in gambling. It's not. Delphi works better than guessing when you have some, but incomplete data - but it's no oracle.

To be fair to Derren Brown, I think he actually said it was just a trick - he was, after all, trained as a lawyer - but a lot of people seem to have missed this.

5 comments:

  1. In defence of Derren Brown, he does make it perfectly clear that he's an illusionist/stage magician/mentalist. He's not some charlatan passing himself off as actually psychic. Anyone so stupid as to imagine that this was anything other than a trick really deserves to lose whatever money they invest in trying to outwit the Lotto.

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  2. Totally agree Brian. I'm a big fan of Derren and was mightily cheesed off that he patently didn't tell us how he got those numbers on to the balls in double quick time. As I sat watching the programme I got more an more worked up as the twaddle went on....

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  3. Thanks to Liz Warrick for pointing out this site http://poeljames.googlepages.com/HowDerrenDidIt.html with a very plausible explanation for the trick - split screen seems an obvious way to do it. It almost had to be a camera trick, rather than a good old fashioned Paul Daniels style trick, because he didn't allow an audience in.

    However, as I mention above, it's not the trick that irritates me, so much as the pretence of the solution. And though, Sara, he does say it is a trick (as I mentioned above), there's still plenty in the programme and on his website about trying it yourself - which to my mind IS a charlatan type thing to say.

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  4. Brian, as you might have seen I've thought long and hard about this and written quite a bit on my blog. I don't think Derren is serious about pretending this is the real solution. He knows it's not true, and he knows that anyone with any sense or grasp of mathematics/probability knows it's not true. See how he dropped in the red herring about rigging the lottery too, an equally ridiculous explanation, and spiced it up by supposedly 'proving' this is what had happened to his studio audience... http://ninethirtyfive.wordpress.com/2009/09/12/derren-brown-lottery-reveal-missing-footage-proves-he-rigged-it/ And I don't doubt there will be more to come. People who believed him are already setting up groups and syndicates - a simple search on facebook and you can find plenty already. A few weeks/months time and if enough people set this up someone could get five numbers and convince even more people Derren was telling the truth. An illusion designed to reproduce itself, like DNA. That's my theory, anyway.

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  5. Niki - yes, I have seen (and enjoyed) your posts on the subject.

    I don't think there's any question that Derren himself knows his 'solution' isn't true, because he did the trick in the first place.

    I think the trouble with the 'anyone with any sense or grasp of mathematics/probability' bit is that the majority of people have a very poor grasp of probability. There are good reasons for this in terms of the way we understand things through patterns etc.

    But the very fact all those people enter the lottery emphasizes how many people don't get it, and so are likely to be misled by a programme that was offered up as 'how I did it' - something that, when you come down to it, is a lie, however many hints are dropped.

    It is entirely legitimate for magicians to use misdirection to avoid people seeing how they work their tricks - but another thing entirely to claim to be revealling how a trick is done, then come up with this kind of stuff.

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