# birthday paradox modified

Not long ago, i wrote an article about the famous birthday paradox. Basically, it says: take an amount of people and calculate the possibility two of them have them same day as their birthday. The possibility is crazy high. Now the basic idea: We calculate the total amount of combination and the amount of different days to get the possibility everybody parties on a different day. We can now subtract from 1 to get the opposite. $m = 365^n$ is the number of all birthday combinations, for $n$ persons. Now we can look at $u = 365 \dotsm (365-n+1) = \frac{365!}{(365-n)!}$. It is the number of combinations for all persons on different days. With Laplace: $\frac{u}{m} = \frac{365!}{(365-n)! \cdot 365^n}$ is the possibility everybodys birthday is on a different day. So we get: $P = 1-\frac{u}{m} = 1-\frac{365!}{(365-n)! \cdot 365^n}$ which is the possibility at least 2 persons have on the same day. Because it's kinda frustrating to calculate this fraction with a calculater i wrote a small python script :)

n=25 # number of persons
ret=1
for i in range(365-n+1,365+1): ret=ret*i
print(1-ret/(365**n))


that helps a lot :) if you plot these data points you get an outcome similar to this:

But that's just the base. A good friend of mine asked me, if we can modify this problem to a larger scale. She wanted to take the year in comparison as well. So i hacked something together to work with a different "year"-range. In fact, it's pretty simple. You just have to apply the year into the calculation. With $y$ as the number of years you get: $m = (365 \cdot y)^n$ is the number of all birthday combinations, for n persons. Now we can look at $u = (365y) \dotsm (365y-n+1)] = \frac{(365y)!}{(365y-n)!}$. It is the number of combinations with all persons on different days. With Laplace: $\frac{u}{m} = \frac{(365y)!}{(365y-n)! \cdot (365 \cdot y)^n}$ is the possibility everybody has on a different day. So we get: $P = 1-\frac{u}{m} = 1-\frac{(365y)!}{(365y-n)! \cdot (365 \cdot y)^n}$ which is the possibility at least 2 persons have on the same day and same year.

n=50 # number of persons
y=25 # range of years
ret=1
for i in range(365*y-n+1,365*y+1): ret=ret*i
print(1-ret/((365*y)**n))


I was stunned to see the outcome...it is incredible...with 50 persons you already get around 12%, which is ridiculous high.

around 120 you even get the 50% which is just....yeah...can't believe it. if you have another basic approach i would love to hear about it :) different outcome as well...i'm still a bit on the: "there has to be something wrong"-side so long :)