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Q: Probabilities ( Answered 5 out of 5 stars,   0 Comments )
Subject: Probabilities
Category: Science > Math
Asked by: mariane0-ga
List Price: $5.00
Posted: 03 Nov 2003 07:57 PST
Expires: 03 Dec 2003 07:57 PST
Question ID: 272161
If you have n balls, all of different colours, and draw one t times, 
at random, 
putting it back afterwards, what is the probability of drawing the 
red ball r times (out of the t times, order without importance)?
Subject: Re: Probabilities
Answered By: mathtalk-ga on 03 Nov 2003 08:32 PST
Rated:5 out of 5 stars
Hi, mariane0-ga:

This is a simple binomial probability.

If we ask for the chance of getting the red ball exactly r times out
of t draws, then the answer is:

C(t,r) * (1/n)^r * (1 - 1/n)^(t-r)

where C(t,r) = t!/(r!(t-r)!) is the number of combinations of t things
taken r at a time (the familiar binomial coefficient).

Please let me know if my answer requires further clarification.

regards, mathtalk-ga
mariane0-ga rated this answer:5 out of 5 stars
Great, just what I needed to know to compare classifier results. When I asked 
this question to people who know about classifiers, they didn't know... :-).
Thank you very much.

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