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5. You roll a pair of fair dice independently. What is the correlation coefficient of the high and low points rolled? Hints:

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X is the low points rolled and Y is the high points rolled.. Then the joint p.m.f is

X

Y

1 2 3 4 5 6
1 1/36 0 0 0 0 0
2 2/36 1/36 0 0 0 0
3 2/35 2/36 1/36 0 0 0
4 2/36 2/36 2/36 1/36 0 0
5 2/36 2/36 2/36 2/36 1/36 0
6 2/36 2/36 2/36 2/36 2/36 1/36

Correlation coefficient(X,Y) =Cov(X , Υ)] Var(X)Var(Y))

= E(XY - E(X) E(Y) (Var(X)Var(Y))

p.m.f of X is

X 1 2 3 4 5 6
P(X) 11/36 9/36 7/36 5/36 3/36 1/36

p.m.f of Y is

Y 1 2 3 4 5 6
P(Y) 1/36 3/36 5/36 7/36 9/36 11/36

Using the joint probability mass function E(XY) =XY x P(XY)

=441/36 = 12.25

Using the probability mass functions of X and Y we get

E(X) =Xх Р(X)

=91/36

E(Y)=Y x P(Y)

=161/36

Var(X) = E(X2)-[E(X)]2

= 301/36-[91/36]2

= 1.971451

Var(Y) = E(Y2)-[E(Y)]2

=764/36-[161/36]2

=1.221451

Covariance(XY) =COV(XY)

=E(XY)-E(X)E(Y)

=441/36-(91/36)X(161/36)

=0.945216

Correlation coefficient(XY) = 0.945216 V1.971451 x 1.221451

= 0.609116

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