Coincidences
“It is a small world, isn’t it? You are on holiday in the Pyrenees. You write a postcard to a friend at home and set off to post it. Then who should you meet but that same friend coming up the street. This not only saves you the cost of a stamp but it also provides a great holiday story – enough to make you think that something spooky is going on.
Similar coincidences happen all the time to someone, somewhere, making the plot-driving inventions of Charles Dickens seem almost plausible. Normally we statisticians deal with the dark underbelly of risk – accidents, deaths, disasters, general gloom and doom – but coincidences show the bright, fun side of the way chance plays out in our lives.
We should perhaps begin by exploring what exactly is a coincidence. It has been defined as a “surprising concurrence of events, perceived as meaningfully related, with no apparent causal connection”. Earlier this year, I invited people to submit examples of surprising concurrences to my website, and looking at over 3,000 of these extraordinary stories, it seems that they tend to fall into certain categories.
Calculating coincidences
There is some nice, fairly simple maths that allows you to work out how many people you need to have a good chance of a match for any characteristic. Suppose that any two people have a 1 in Cchance of matching – for example, for an exact birthday match, C = 365. Then to have a 50% chance of a match in a group of N people, it turns out that N needs to be around 1.2 √C. For a birthday match, this means that we need around 1.2 √365 = 23 people.” (from the BBC – Read more here.)
