The Science of Discworld Revised Edition

sources of bias that they could think of. The ‘selective reporting’ was done by researchers who had contrary evidence but didn’t publish it because they thought it must be wrong, by journal referees who accepted papers that confirmed a decline more often than they accepted those that didn’t, and by the press – who strung together a whole pile of sex-related defects in various parts of the animal kingdom into a single seamless story, unaware that each individual instance has an entirely reasonable explanation that has nothing to do with falling sperm-counts and often nothing to do with sex.
    Sexual abnormalities in fish near sewer outlets, for instance, are probably due to excess nitrites, which all fish-breeders know cause abnormalities of
all
kinds – and not to oestrogen-like compounds in the water, which would bolster the ‘sperm count’ story. Current data from fertility clinics, by the way, show no signs of a decline.
    Humans add narrativium to their world. They insist in interpreting the universe as if it’s telling a story. This leads them to focus on facts that fit the story, while ignoring those that don’t. But we mustn’t let the coincidence, the clump, choose the sample space – when we do that, we’re ignoring the surrounding space of near-coincidences.
    Jack and Ian managed to test this theory on a trip to Sweden. On the plane, Jack predicted that a coincidence would happen at Stockholm airport – for reasons of selective reporting. If they looked hard enough, they’d find one. They got to the bus stop outside the terminal, and no coincidences had occurred. But they couldn’t find the right bus, so Jack went back to the enquiries desk. As he waited, someone came up next to him – Stefano, a mathematician who normally occupied the office next door to Jack’s. Prediction confirmed. But what was really needed was evidence of a near-coincidence – one that hadn’t happened, but could have been selectively reported if it had. For instance, if some other acquaintance had shown up at exactly the same time, but on the wrong day, or at the wrong airport, they’d never have noticed. Near coincidences, by definition, are hard to observe … but not impossible. Ian happened to mention all of the above to his friend Ted, who was visiting soon after. ‘Stockholm?’ said Ted: ‘When?’ Ian told him. ‘Which hotel?’ Ian told him ‘Funny, I was staying there
one day later than you!
’ Had the trip been one day later, the ‘coincidental’ encounter with Stefano wouldn’t have happened – but the one with Ted would.
    What we must not do, then, is to look back at past events and find significance in the inevitable few that look odd. That is the way of the pyramidologists and the tea-leaf readers. Every pattern of raindrops on the pavement is unique. We’re not saying that if one such patterns happens to spell your name, this is not to be wondered at – but if your name had been written on the pavement in Beijing during the Ming dynasty, at midnight, nobody would have noticed. We should not look at
past history
when assessing significance: we should look at all the other things that might have happened instead.
    Every event is unique. Until we place that event in a category, we can’t work out which background to view it against. Until we choose a background, we can’t estimate the event’s probability. If we consider the sample space of all possible DNA codes, for instance, then we can calculate the probability of a human being having exactly
your
DNA code – which is vanishingly small. But it would be silly to conclude that it is impossible for you to exist.
    1 Indeed, it is a fundamental part of story telling. If the hero did not overcome huge odds, what would be the
point?
    2 Possibly he was holding a large axe at the time.

THIRTY-FIVE
STILL BLOOD LIZARDS

    ‘ THE FUTURE IS lizard,’ said Ridcully. ‘Obviously.’
    It was a few days later. The omniscope was focused on a mound of leaves and rotting vegetation a little way from the banks of the river. There was a large depression hanging over the Senior Wrangler, and the Dean had a black eye. The war between land and sea had just entered a terminal stage.
    ‘Little portable seas,’ said Ponder. ‘You know, I never thought of them like that.’
    ‘An egg is an egg, however you look at it,’ said Ridcully. ‘Look, you two, I don’t want to see a scuffle like that again, d’you hear?’
    The Senior Wrangler dabbed at