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May 9th, 2008, 06:30 PM  #1 
Newbie Joined: May 2008 From: Tule reed infested malarial swamp Posts: 1 Thanks: 0  How do I calculate chance of running into a certain person?
Hello all, I want to calculate the chances of running into a certain individual in the city I live in. I used population, square feet of solid land area, and other factors but things became too complicated to be correct; There must be a simpler formula for this. Your insight would be appreciated! 
May 10th, 2008, 06:24 AM  #2 
Global Moderator Joined: Nov 2006 From: UTC 5 Posts: 16,046 Thanks: 938 Math Focus: Number theory, computational mathematics, combinatorics, FOM, symbolic logic, TCS, algorithms 
Estimate the number of different people you see each week. Then estimate how many of them you see at least a few times a year. The difference can be seen as the number of people you see 'randomly'. Making the (bad) assumption that the chance you'll see someone other than regularly is the same for any person across the city, you have only to divide this number by the city's population to get a first approximation of the chance you'll see someone. Perhaps I see 300 people in a given week, but 150 of these I see regularly (coworkers, family, etc.). If my city has a population of half a million, then the chance I see someone (other than someone I see regularly) in the city in a given week can be approximated by 150/500000 or 0.03%. The chance I'd see someone from the city in a given year would then be about 1.5%. Of course depending on how much interaction is needed to count as "running into" the odds could differ quite a lot. Also, if you were looking over larger timespans you'd need different formulas  this one's only good for small percentages (and even then a bit questionable). 

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