The probability of 2 people having the same birthday is 1 in 365 because it’s the same as picking person A’s birthday as a specific day in the year and checking whether person B has their birthday on that date.
Now, the reason the number is so low is that you are basically comparing pairs and with 23 people there are 253 different pairings (23 choose 2 or 22*23/2). With each pair having a 1/365 chance to have the same birthday and having 253 distinct pairs, you would have to fail a 1/365 check 253 times in a row. The formula you can use for the success rate is 1 - (1-p)^x with p being the probability and x the number of trials, so in this case
1 - (1 - 1/365)^253 = 0.5004
In essence, the unintuitive part of the “paradox” is how fast the number of possible pairs grows the more people you add.
The probability of 2 people having the same birthday is 1 in 365 because it’s the same as picking person A’s birthday as a specific day in the year and checking whether person B has their birthday on that date.
Now, the reason the number is so low is that you are basically comparing pairs and with 23 people there are 253 different pairings (23 choose 2 or
22*23/2). With each pair having a 1/365 chance to have the same birthday and having 253 distinct pairs, you would have to fail a 1/365 check 253 times in a row. The formula you can use for the success rate is1 - (1-p)^xwith p being the probability and x the number of trials, so in this case1 - (1 - 1/365)^253 = 0.5004In essence, the unintuitive part of the “paradox” is how fast the number of possible pairs grows the more people you add.