What are the odds of two people sharing the same numerical day in different months as a birth date?

Question

For example, my birthday is July 26, 1977. My half-brother's birthday is April 26, 1995. What are the odds of two people, related or not, being born on the 26th day of any month?

Answer 1

If an approximate answer satisfies you: The probability is *about* 1 in 30. (There are about 30 days in a month, so the odds of two people having the same day number are about 1 in 30.)

The *exact* probability cannot really be computed, because not every birthday is as common as every other one. (For example, in the United States, September and April are the most common birth months, in that order--and doubtless some days are more common for birthdays than others, simply because people have more babies on some days than others.)

If you make the simplifying assumption that each day of a year is as likely as any other to be a birthday, then you can compute the probability as follows. (Note that here are an average of 365.2425 days per year; 365 days in every year, and an averag of 0.2425 leap days per year.)

Probability of one person being born on a specific day, numbered 1-28: There are 12 of each of these days per year, with an average of 365.2425 days per year. Probability: 12/365.2425

Probability of one person being born on the 29th: There are 11.2425 months with a 29th, on average. Probability: 11.2425/365.2425.

Probability of one person being born on the 30th: There are 11 months with a 30th. Probability: 11/365.2425.

Probability of one person being born on the 31st: There are 7 months with a 31st. Probability: 7/365.2425.

Now, to compute the probabilities for two people, I will divide up into cases based on what day of the month Person 1 was born.

Probability of Person 1 being born on a day from 1-28, and Person 2 also being born on that same day: (12/365.2425)^2.

Probability of Person 1 being born on the 29th of a month, and Person 2 also: (11.2425/365.2425)^2.

Probability of Person 1 being born on the 30th of a month, and Person 2 also: (11/365.2425)^2.

Probability of Person 1 being born on the 31st of a month, and Person 2 also: (7/365.2425)^2.

Probability of both people having the same day as a birthday: 28 * (12/365.2425)^2 + (11.2425/365.2425)^2 + (11/365.2425)^2 + (7/365.2425)^2.

This is about 3.24462% chance, which is just a bit less than 1/30. (1/30 is about 3.33333%.)

Answer 2

Not sure of this particular question, however this something related that you'll find interesting.

Probability people will appr
How many people does it take so that the probability of two of those people having the same birthday (assuming that all people in the room are born in the same year) is 1 (all probabilities are represented by a number x where 0 <= x <= 1).

Answer: 23 people.

So based on that i can assume that the likelyhood of two people having the same numerical day are pretty low (Statistically speaking, as you are increasing the probability range from 1 in 365 days to 12 in 365 days)

Answer 3

Your question is specifically the 26th.
There are 12 26ths in the year, out of 365 days. Forget leap years.

So P(any one being born on the 26th) = 12/365
P(two people being born on 26th) = (12/365)^2

Now if you want the prob of them being born on any same date, then let's do the following.
The prob will be (12/365)^ for each day from the 1st to the 28th.
It will be (11/365)^2 for the 29th and 30th.
It will be (7/365)^2 for the 31st.
So adding them all, we get 4323/133425
= 1:30.8

Answer 4

I don't know the odds but I have a friend who was born on the same day one year apart and the same state as someone he knew ONLY after the two at different times moved to my state and started working for the same company in which I worked for.

Answer 5

Well i dunno what you wanted but My b day is July 9th and so is my hubby's just a different year. My daughter was born on May 13th my youngest son was also born on May 13th 5 yrs later.Oldest daughter June 12th niece November 12th same year. Not big on coincidences, but somethings up. I'd say the odds are very good!

Answer 6

I have no clue how you would find that answer.

My mom and dad's BD is on the same day 3/21, but they are 2 yrs. apart. My BD is 7/30 and my husbands is 5/30.

Answer 7

i think its:

12 to 365

it would be 12 days that have the date 26 a year, which is 365 so it would be 12:365
if you want smaller odds its about
1 to 30.416667

if its a leap year (366 days) it would be
12 to 366
or
1 to 30.5

Answer 8

Since there are 12 such days in an year, 12/365 is the probability.

Answer 9

Probability = Number of Favorable Events/Number of Total Events

Answer 10

I DO NOT KNOW BUT I KNOW MY FRIEND WE GREW UP FOUR HOUSE AWAY FORM EACH OTHER WE HAD THE SAME BIRTHDAY AND THERE IS 3 PEOPLE FORM WORK WHO HAVE THE SAME BIRTHDAY DIFFERENT YEARS SO IT SHOULD BE I OUT OF 30