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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


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[(YE[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[(XMean)^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?)



