| MadSci Network: Other |
The probability that one person will have the same birthday (month and day of the month) will be 1/365.25. If you also put the year of birth into the calculation, the chances will depend on the range of ages in the group you are thinking about. If you are thinking about just students, the probability of two persons having the same birth date will be smaller than 1/365.25 maybe 4-10 times as small. It could be even smaller if you were picking people randomly from registered voters, perhaps 40-50 times smaller than 1/365.25. The probability of having the same name will depend on the group, but one way to get a guess would be to see how many different names there are in a phonebook or a student registry. (Again it depends on who you are picking from.) This is really an experimental question. Go to a large phone book, take 20 pages at random and count the number of distinct names. Divide one by that number. When thinking about probability of two things happening at once, you multiply tthe numbers. So you might decide that the probability of having the same birthdate (year, month and day) was 1/3,000 and the probability of having the same name was 1/20,000 so the probability of having the same name AND the same birthday would be 1/3,000 times 1/40,000 or 1/120,000,000. If you have a name like Winsemius, the odds are much greater against any matches. I suspect I am the only David Winsemius with my birthday on the planet Earth. That is 1/3,000,000,000. But anyone will tell you that I am not typical. David Winsemius
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