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Topic: Ball problem
Replies: 1   Last Post: Jan 28, 2008 9:51 PM

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Ladnor Geissinger

Posts: 313
From: University of North Carolina at Chapel Hill
Registered: 12/4/04
Re: Ball problem
Posted: Jan 28, 2008 9:51 PM
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Expected Value of any Random Variable Y is computed as
E[Y] = SUM(Y[outcome]*Prob[outcome], all outcomes in sample space), and
Variance[Y]=E[(Y-E[Y])^2]

So for your X, Mean=E[X]=(1+2)*(1/3)+(1+3)*(1/3)+(2+3)*(1/3)=4, and
Variance[X]=E[(X-Mean)^2]=((-1)^2)*(1/3)+0*(1/3)+(1^2)*(1/3) =2/3.

> Hello, my name is Jerry Katzman MD and I would really
> need some help with this problem:
> There are 3 balls (1,2,3) in a box. You select 2 at
> random, w/o replacement and count the total. The
> samplE space is ( (1,2) (1,3) (2,3) ). If X is the
> total of the two balls, the variance is.......
> (It seems easy, but I get 1 and the answer giver is
> 2/3 - What am I missing?)



Date Subject Author
1/28/08
Read Ball problem
Jerry Katzman
1/28/08
Read Re: Ball problem
Ladnor Geissinger

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