Birthdays of grandfather and granddaughter

  • Context: Undergrad 
  • Thread starter Thread starter frecklesan
  • Start date Start date
Click For Summary

Discussion Overview

The discussion centers around calculating the probability of a granddaughter being born on the same date and time as her grandfather. It explores various interpretations of the question, assumptions about birth timing, and the mathematical setup required for such a probability calculation.

Discussion Character

  • Mathematical reasoning
  • Exploratory
  • Debate/contested

Main Points Raised

  • One participant seeks help in determining the probability of a granddaughter being born on the same date and time as her grandfather, expressing uncertainty in how to approach the problem.
  • Another participant suggests that the question is vague and could be interpreted in multiple ways, proposing a simple probability of 1 in 365 for the same day.
  • Some participants note that assumptions about life spans, parental ages, and time zones could significantly affect the calculations.
  • A participant proposes a flat probability distribution for the birth date, hour, minute, and second, providing a breakdown of the probabilities for each time unit.
  • One participant calculates the probability of being born on the same month, day, and time as the grandfather, expressing appreciation for the discussion.
  • Another participant suggests calculating the probability of not being born in the same time period and then subtracting from one, relating it to the birthday problem in probability theory.
  • A later reply clarifies that with only two individuals involved, the calculation is simpler than the classic birthday problem involving larger groups.

Areas of Agreement / Disagreement

Participants express differing views on how to interpret the problem and the assumptions necessary for calculating the probability. No consensus is reached on a definitive method or outcome.

Contextual Notes

Participants highlight the need for specific assumptions regarding birth timing and distribution, which remain unresolved. The discussion reflects varying interpretations of the problem's parameters.

frecklesan
Messages
3
Reaction score
0
Hi to all you smart people out there: I need your help, and don't know how to solve this. If a grandfather is born on February 4, 1950 at ll:07 a.m. and his granddaughter is born on February 4, 2002 at ll:07 a.m. how do I find how the probability or stats of this happening between birthdays of grandfather and granddaughter. I really appreciate your help. Thank you very much.
 
Physics news on Phys.org
do you mean the same day, hour, minute or what. the question as posed is too vague and has many possible interpretations. the simplest is, given your grandfather's DoB, then there is a chance of 1 in 365 that your granddaughter will be born on the same day in the year
 
It also depends on what kind of assumptions you make about average life spans, average age of parents at the time of giving birth, whether you include all time zones, and various other things.

If you have a notion of how precisely the two are aligned, and you have a probability function, then you can start using numbers.
 
Thanks Matt, I guess I am asking what are the chances that a grandaughter would be born on the same birthdate, and time as the grandfather? I don't know how to setup the problem. He is born 2/4 and she is born on 2/4 both at ll:07 a.m. I am not very good at math or expressing myself. Thank you.
 
freclesan - If you assume that the probability distribution is flat (which is not necessarily valid), then the answer is:
1/365 for the right day (since 2002 is not a leap year)
1/(365*24) for the right hour
1/(365*24*60) for the right minute
1/(365*24*60*60) for the right second
and so on.
 
Hi, so then if I multiply 1/(365*24*60) = 525600 (I don't think they know the seconds) then it means there is a l in 525600 of his being born on the same month, day and time (hour plus minutes) as his granddaugher, right? I really appreciate this very much. You people have made my day!
 
But what you have to do is work out the probability they are not born in the same time period and then take 1 minus that. It's just the old what's the probability there are two people in a class with the same birthday. For a class of 25 the probability is 50%.
 
Originally posted by selfAdjoint
But what you have to do is work out the probability they are not born in the same time period and then take 1 minus that. It's just the old what's the probability there are two people in a class with the same birthday. For a class of 25 the probability is 50%.

Actually, the probability for a random group of 22 people to have a pair with the same birthday is already better than 50%.

Since there are only two people in this problem - the daughter and the grandfather, it's a bit simpler.
 

Similar threads

  • · Replies 1 ·
Replies
1
Views
3K
  • · Replies 6 ·
Replies
6
Views
5K
  • · Replies 4 ·
Replies
4
Views
3K
  • · Replies 145 ·
5
Replies
145
Views
18K
  • · Replies 34 ·
2
Replies
34
Views
7K
  • · Replies 22 ·
Replies
22
Views
2K
  • · Replies 5 ·
Replies
5
Views
2K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 24 ·
Replies
24
Views
2K
  • · Replies 9 ·
Replies
9
Views
2K