The pin code problem

How many people do you need to get together before it will be reasonably likely that two share the same pin code? We know by the pigeonhole principle that given more than 10,000 we can be certain there is a repeat, but does the probability pass 50% much earlier?

It turns out that this is closely related to the famous “birthday problem” and that you actually need fewer than 150 people before it is reasonably likely two share the same pin. In fact, if you include just SOME of the data we have on actual pin codes people selected, the number is dramatically smaller! You can read the details over on my personal blog: