# Answer to Question #15611 in Statistics and Probability for abid khan

Question #15611

Conditional Probabilities:Select all of the following statements that are true for all joint distributions over X and Y

P(x,y)=P(x)P(y)

P(x,y)=P(x|y)P(y)

P(x,y)=P(x|y)P(y|x)

P(x)=∑yP(x|y)

P(x)=∑yP(x,y)

None of the above.

P(x,y)=P(x)P(y)

P(x,y)=P(x|y)P(y)

P(x,y)=P(x|y)P(y|x)

P(x)=∑yP(x|y)

P(x)=∑yP(x,y)

None of the above.

Expert's answer

Only statement (2) and (5) are true in general.

(1) By definition holds

if X and Y are independent

(2) P(x,y)=P(x|y)P(y) this is definition of

P(x|y):

P(x|y) = P(x,y)/P(y)

(3) the expression P(x|y)P(y|x) has

no sense

(5) ∑yP(x,y) means the sum over all y of values P(x,y). This sum

is equal to P(x)

(4) ∑yP(x|y) means the sum over all y of

P(x|y)=P(x,y)/P(y):

∑yP(x|y) = ∑y P(x,y)/P(y)

again this sum has

no sense

(6) None of the above.

This is also not true, since (2)

and (5) are correct

(1) By definition holds

if X and Y are independent

(2) P(x,y)=P(x|y)P(y) this is definition of

P(x|y):

P(x|y) = P(x,y)/P(y)

(3) the expression P(x|y)P(y|x) has

no sense

(5) ∑yP(x,y) means the sum over all y of values P(x,y). This sum

is equal to P(x)

(4) ∑yP(x|y) means the sum over all y of

P(x|y)=P(x,y)/P(y):

∑yP(x|y) = ∑y P(x,y)/P(y)

again this sum has

no sense

(6) None of the above.

This is also not true, since (2)

and (5) are correct

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