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Topic: say I flip a coin 100 times...
Replies: 46   Last Post: Apr 18, 2005 9:42 PM

 Messages: [ Previous | Next ]
 Ralph Hartley Posts: 117 Registered: 12/8/04
Re: say I flip a coin 100 times...
Posted: Apr 13, 2005 10:35 AM

ken quirici wrote:
> There are 1M people and 100 coins. There are 2**100 possible
> sequences of 100 coins. The chance that 1 out of the 1M will flip
> the same sequence as you is (with ** indicating exponentiation)
>
> 1M * (2**-100) (if I remember correctly you add the
> probabilities of independent events
> to get the probability that they will
> all occcur).

You *multiply* the probabilities of independent events to get the
probability that they will all occur. (What you said was wrong, but what
you *did* was correct).

You add the probabilities of mutually exclusive events to get the
probability that one of them will occur.

The events here are not quite mutually exclusive. Your answer is off by
the probability that more than one sequence will match (around (1M *
(2**-100))**2).

Ignoring that is safe for practical purposes, but then so is ignoring
the probability of one match. Both are zero for almost all practical
purposes.

Ralph Hartley

Date Subject Author
4/12/05 HERC777
4/12/05 Jan Burse
4/12/05 HERC777
4/12/05 George Greene
4/13/05 Barb Knox
4/13/05 HERC777
4/13/05 Yajun
4/13/05 HERC777
4/13/05 Jay
4/13/05 HERC777
4/13/05 Jay
4/14/05 Yajun
4/14/05 George Greene
4/14/05 George Greene
4/13/05 Will Twentyman
4/13/05 briggs@encompasserve.org
4/13/05 briggs@encompasserve.org
4/14/05 George Greene
4/14/05 tchow@lsa.umich.edu
4/14/05 briggs@encompasserve.org
4/12/05 Tony
4/13/05 HERC777
4/13/05 ken quirici
4/13/05 Ralph Hartley
4/13/05 Robert Low
4/13/05 Ralph Hartley
4/13/05 Robert Low
4/13/05 tchow@lsa.umich.edu
4/13/05 HERC777
4/13/05 tchow@lsa.umich.edu
4/14/05 HERC777
4/14/05 George Greene
4/14/05 HERC777
4/15/05 tchow@lsa.umich.edu
4/15/05 tchow@lsa.umich.edu
4/16/05 George Greene
4/16/05 HERC777
4/17/05 George Greene
4/17/05 Patricia Shanahan
4/17/05 HERC777
4/18/05 Patricia Shanahan
4/17/05 tchow@lsa.umich.edu
4/18/05 HERC777
4/18/05 George Greene
4/18/05 tchow@lsa.umich.edu
4/18/05 HERC777