The mathematics of running into people again

The last weekend in Cartagena I got several new great friends. One of these was an Italian guy who just moved to Cartagena to study there for 10 months. He took my phone number and we decided to meet up someday in the future. The next day I was sitting at a restaurant eating tapas in this 200 000+ inhabitants city, when the same Italian guy walked past and looking in through the door. I was a little bit surprised but not too much as these kinds of things happen to me a lot. Lately I have started to wonder if this also is common to other people.

The restaurant where we found each other again was not near any special place nor near where we were the night before. Not either did we have any common interest that could have drawn us both to this area at exact this time. It was just totally random as I see it.

As I now know around 10 people in Cartagena, this should mean that the chance of running into one of those friends are 1 in 20 000. I have only been to this city 6 days in total, which also should mean that every one of these days is a chance of 1 of 3 333 that I run into somebody I know (from that city, if everybody you see is a new face to you). This usually happens to me in a lot of cities, not only this one time. In other case I would also calculate all cities this don’t happen in, to get a lower number.

In a regular active day I estimate a normal person who are in the city center see about 500 detailed faces in a day (this was an empty Sunday, but lets ignore that). That would mean that my 3 333 now is down to 1 in 7, not so impressive anymore.

Of course there are a LOT of more factors, like how good your face memory and eye vision is, how tired or thoughtful you are, if you look at shop windows or people, how many hours a week you are in certain central areas, if you are indoors or outdoors, how many people are around you, if you are sitting still or moving, your and the other persons age, how good looking or interesting they are, where you work, where you live and so on.


Bottom note: This also happens to me in much bigger cities, for example in London, where I also know even fewer people. And once I ran into a girl in another country than I first met her in (on the same trip, without knowing we were both traveling in the same direction). If you have similar stories, please share them as comments here below!

Ocean of people near the harbor in Cartagena, at the Sailing MedCup Circuit
Ocean of people near the harbor in Cartagena, at the Sailing MedCup Circuit

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