What are the odds of two random people in the world meeting?

[jellybread]

As the subject says, I am trying to figure out what the odds of two random people in the world meeting each other sometime in their lives would be if we use 7 billion as the worlds population. Thank you very much for reading.

 

[tkhunny]

Re: What are the odds of two random people in the world meet

1) Zero. There are no random people in the world. Good try, though.

2) Zero. Everyone who is able to get out an meet people has made at least one conscious choice. Whoops, there went your randomness.

3) One. If they just met, well, obviously...

 

[jellybread]

Re: What are the odds of two random people in the world meet

Ok, so if I remove the word random from the question can anyone help me figure this out? I'm trying to determine the odds of any two people in the world meeting each other at some point in their lifetimes assuming the world's population is 7 billion.

The closest I've been able to find is at this link: http://answers.yahoo.com/question/index ... 917AAFdb2U where someone states the odds of two people living in the United States meeting each other as 1 in 28,000. I have no idea how he came to this conclusion or if its even accurate. Any ideas?

 

[tkhunny]

Re: What are the odds of two random people in the world meet

No, no, the word "random" is extremely important. It's a whole different calculation without that. It is common to pose such questions badly. Such as, "What's the probability that you and I will meet?" Well, if we never go anywhere or do anything that is similar or in the same place, it's close to zero. Try this, "Two people are randomly selected from the present population of Earth. What is the a priori probability that they will meet? This is still not a great question, since the population of Earth is not static. We'll have to assume that we have 6 billion and this number always is the same people! Now, look up the "same birthday" problem and see if you can adapt it to 6 billion people (rather than say, 23) with infinitely many places to meet (rather than 365). You may wish to define "meet". Maybe a small enough surface area where one could not be avoided. Perhaps a "Zone of Influence" defined by a circular area with 12 ft (~3.7 m) radius.