# joint and marginal PMF's

• Aug 26th 2010, 09:39 PM
ulysses123
joint and marginal PMF's
X
-1 0 1
-1 0 0 (1/3)
Y 0 0 (1/3) 0
1 (1/3) 0 0

Now for this i am pretty sure my calculations are correct but i want to check because there seems to be some errors in the textxbook which confused me.

for the marginal PMF say of x, i simply thought i simply sum the coresponding column.
For example p{-1)=1/3
p(0)=1/3
p(1)=1/3

But when they do this in the book the value they obtain does not corespond to the sum of the column they are refering to, and this happens twice, and i dont think there would be two errors in the same question and hence i'm a bit confused now.

Next part to find E[XY] i multiply all values of x and y which gives me XY=-1,0,1, with corresponding probabilities.

Then to check if these random variables are independendant i check if E[XY]=E[x]E[Y]
which in this case is zero. so they must be independant. Can someone tell me if i am doing something wrong.
• Aug 27th 2010, 10:19 PM
matheagle