The Creativity Code. Marcus du SautoyЧитать онлайн книгу.
In each round, the rejected queens propose to the next king on their list and the kings always go for the best offer they receive. In this third round the rejected Queen of Diamonds proposes to the King of Diamonds (who has been standing like that kid who never gets picked for the team). Despite the fact that the Queen of Diamonds is low down on his list, he hasn’t got a better option, as the other three queens prefer other kings who have accepted them.
Third round
Finally everyone is paired up and all the marriages are stable. Although we have couched the algorithm in terms of a cute parlour game with kings and queens, the algorithm is now used all over the world: in Denmark to match children to day-care places; in Hungary to match students to schools; in New York to allocate rabbis to synagogues; and in China, Germany and Spain to match students to universities. In the UK it has been used by the National Health Service to match patients to organ donations, resulting in many lives being saved.
And it is building on top of the puzzle solved by Gale and Shapley that the modern algorithms which run our dating agencies are based. The problem is more complex since information is incomplete. Preferences are movable and relative, and shift even within relationships from day to day. But essentially the algorithms are trying to match people with preferences that will lead to a stable and happy pairing. And the evidence suggests that the algorithms could well be better than leaving it to human intuition.
You might have detected an interesting asymmetry in the algorithm that Gale and Shapley cooked up. We got the queens to propose to the kings. Would it have mattered if we had invited the kings to propose to the queens instead? Rather strikingly it does. You would end up with a different stable pairing if you applied the algorithm by swapping kings and queens.
The Queen of Diamonds would end up with the King of Hearts and the Queen of Clubs with the King of Diamonds. The two queens swap partners, but now they’re paired up with slightly lower choices. Although both pairings are stable, when queens propose to kings, the queens end up with the best pairings they could hope for. Flip things around and the kings are better off.
Medical students in America looking for residencies realised that hospitals were using this algorithm to assign places in such a way that the hospitals did the proposing. This meant the students were getting a worse deal. After some campaigning by students who pointed out how unfair this was, eventually the algorithm was reversed to give students the better end of the deal.
This is a powerful reminder that, as our lives are increasingly pushed around by algorithms, it’s important to understand how they work and what they’re doing, because otherwise you may be getting shafted.
The battle of the booksellers
The trouble with algorithms is that sometimes there are unexpected consequences. A human might be able to tell that something weird was happening, but an algorithm will just carry on doing what it was programmed to do, regardless of how absurd the consequences may be.
My favourite example of this centres on two second-hand booksellers who ran their shops using algorithms. A postdoc working at UC Berkeley was keen to get hold of a copy of Peter Lawrence’s book The Making of a Fly. It is a classic published in 1992 that developmental biologists often use, but by 2011 the text had been out of print for some time. The postdoc was after a second-hand copy.
Checking on Amazon, he found a number of copies priced at about $40, but then was rather shocked to see a copy on sale for $1,730,045.91. The seller, profnath, wasn’t even including shipping in the bargain. Then he noticed that there was another copy on sale for even more! This seller, bordeebook, was asking a staggering $2,198,177.95 (plus $3.99 for shipping of course).
The postdoc showed this to his supervisor, Michael Eisen, who presumed it must be a graduate student having fun. But both booksellers had very high ratings and seemed to be legitimate. Profnath had had over 8000 recommendations over the last twelve months, while bordeebook had had over 125,000 during the same period. Perhaps it was just a weird blip.
When Eisen checked the next day to see if the prices had dropped to more sensible levels, he found instead that they’d gone up. Profnath now wanted $2,194,443.04 while bordeebook was asking a phenomenal $2,788,233.00. Eisen decided to put his scientific hat on and analyse the data. Over the next few days he tracked the changes in an effort to work out if there was some pattern to the strange prices.
Eventually he spotted the mathematical rule behind the escalating prices. Divide the profnath price by the bordeebook price from the day before and you always got 0.99830. Divide the bordeebook price by the profnath book on the same day and you always got 1.27059. Each seller had programmed their website to use an algorithm that was setting the prices for books they were selling. Each day the profnath algorithm would check the price of the book at bordeebook and would then multiply it by 0.99830. This algorithm made perfect sense because the seller was programming the site to slightly undercut the competition at bordeebook. It is the algorithm at bordeebook that is slightly more curious. It was programmed to detect any price change in its rival and to multiply this new price by a factor of 1.27059.
The combined effect was that each day the price would be multiplied by 0.99830 × 1.27059, or 1.26843. This ensured that the price would grow exponentially. If profnath had set a sharper factor to undercut the price being offered by bordee-book, you would have seen the price collapse over time rather than escalate.
The explanation for profnath’s algorithm seems clear, but why was bordeebook’s algorithm set to offer the book at a higher price? Surely no one would buy the more expensive book? Perhaps they were relying on their bigger reputation with a greater number of positive recommendations to drive traffic their way, especially if their price was only slightly higher, which at the start it would have been. As Eisen wrote in his blog, ‘this seems like a fairly risky thing to rely on. Meanwhile you’ve got a book sitting on the shelf collecting dust. Unless, of course, you don’t actually have the book …’
Then the truth suddenly dawned on him. Of course. They didn’t actually have the book! The algorithm was programmed to see what books were out there and to offer the same book at a slight markup. If someone wanted the book from their reliable bordeebook’s website, then bordeebook would go and purchase it from the other bookseller and sell it on. But to cover costs this would necessitate a bit of a markup. The algorithm thus multiplied the price by a factor of 1.27059 to cover the purchase of the book, the shipping and a little extra profit.
Using a few logarithms it’s possible to work out that the book most likely first went on sale forty-five days before 8 April at about $40. This shows the power of exponential growth. It only took a month and a half for the price to reach into the millions! The price peaked at $23,698,655.93 (plus $3.99 shipping) on 18 April, when finally a human at profnath intervened, realising that something strange was going on. The price then dropped to $106.23. Predictably bordeebook’s algorithm offered their book at $106.23 × 1.27059 = $134.97.
The mispricing of The Making of a Fly did not have a devastating impact for anyone involved, but there are more serious cases of algorithms used to price stock options causing flash crashes on the markets. The unintended consequences of algorithms is one of the prime sources of the existential fears people have about advancing technology. What if a company builds an algorithm that is tasked with maximising the collection of carbon, but it suddenly realises the humans who work in the factory are carbon-based organisms, so it starts harvesting the humans in the factory for carbon production? Who would stop it?
Algorithms are based on mathematics. At some level they are mathematics in action. But they don’t really creatively stretch the field. No one in the mathematical community feels particularly threatened by them. We don’t really believe that algorithms will turn on their creators and put us out of a job. For years I believed that these algorithms would do no more than speed up the mundane part of my work. They were just more sophisticated