I have always been surprised how often I have been in a small group with at least two people sharing the same birthday (it’s happened at school, work, on holiday, on projects, basically more times than I can remember). With 365 days to pick from, it seems logical to think you’d need a pretty large group before duplications start to occur…

…but it takes just 23 people for there to be a greater than 50% chance that two people in the group will share the same birthday. For a good explanation of the maths behind the fact and why we so easily get it wrong, read The Law of Small Errors.

I find this simple probability exercise serves as a good reminder to test assumptions when analysing statistics. Sometimes the answer we expect to see is not remotely close to reality, but our brains have an annoying habit of preferring to believe expected myths rather than actual truths, no matter what evidence is presented before us. As we start to integrate business intelligence and performance dashboards into everyday activities, we need to beware our tendency to draw the wrong conclusions from the information presented.

I’ve read a few good books that cover this subject recently, including:

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