Coincidence: A Novel by J. W. Ironmonger Page A

morning, his arm in a plaster cast. He had an ordinary day. He taught a class on the social understanding of unusual events. It was one of his fields of expertise. Most of us, he argued, are bad at calculating the likelihood of quite normal events and we tend, therefore, to see them as remarkable, or divinely inspired. ‘How likely is it,’ he asked his class of twenty-five students, ‘that you visit the theatre, and there in the audience is someone you just happen to know? Or let us say you stop at a motorway service station, or you go on holiday to Ibiza and there, in the same hotel, is someone you knew from school?’ He let the students ponder this. ‘I once flew to Madrid,’ he said, ‘and on the same flight were a couple whose wedding I had attended a year before. Was that a coincidence?’
    He took a marker and did some calculations on a whiteboard. ‘How many Facebook friends do you have?’ he asked a girl in the front row.
    She giggled. ‘Around, maybe, four hundred,’ she said.
    â€˜OK.’ Thomas wrote 400 . ‘Now if we add to these all your relatives, and your neighbours and people you’ve lost touch with, then four hundred probably becomes six hundred, doesn’t it? Add all the people you’d recognise from school or university – not friends, just people you know – let’s make it a thousand.’ He crossed out the 400 and wrote 1,000 . ‘When we look at it, that turns out to be a good estimate for the number of friends and relatives and acquaintances that each of us will typically have. So you’d probably recognise and go and greet a thousand people if you were to meet them in an unexpected place like an airport lounge or on a beach.’ Again he sketched out the calculation on the whiteboard. ‘How often are we in any kind of situation where there are two or three hundred strangers? At a theatre? In a supermarket? On a tube train? In the high street? Maybe four or five times a day.’ He wrote the numbers up. ‘Now your thousand acquaintances represent, say, one person in every fifty thousand in the UK. But you probably see the faces of fifteen hundred people every day. So you should have at least one chance encounter with somebody you know at least once a month.’ Thomas lowered himself carefully onto the desk at the front of the lecture theatre. His arm was in a sling and it was still uncomfortable.
    â€˜But that assumes,’ he went on, ‘that we’re all randomly distributed around the country. In reality we can ignore big chunks of the population. We don’t need to count children, or the very elderly, or stay-at-home farmers, or the housebound, or people in prison, or anyone you simply wouldn’t bump into on a train from London. That should increase your likelihood of a chance encounter to one every fortnight. So the next time you meet a friend in Covent Garden, don’t say, “What a coincidence meeting you here!” Because it isn’t.’
    â€˜So what are the chances,’ he asked them, ‘that two of us will share the same birthday?’
    Many of the students had encountered this conundrum before. A quick poll of birthdays was held. In a gathering of twenty-three people, Thomas knew, there is more than a fifty per cent likelihood that two will share a birthday. The maths is counter-intuitive; most people would consider it a great coincidence if two guests at a party shared a birthday. In fact, with the twenty-five students in the room, it would be more unusual if no birthdays were shared. As each student called out his or her birthday, there was an air of expectation; but no two matched. Thomas held up his hand. ‘There’s one more person left,’ he said, and he pointed to himself. ‘My birthday is on the thirtieth of June.’ There was a gasp and some applause. ‘Who shares my birthday?’ asked Thomas. ‘Can you please stand