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Homework Help: How to work out Covariance ?

  1. May 25, 2008 #1
    1. The problem statement, all variables and given/known data

    I have a joint pdf f_{XY}(x,y) = (2+x+y)/8

    and I have to find cov(x,y)
    2. Relevant equations
    Now I know the formula is cov(X,Y)=E((X-mu)(Y-v)) where E(X)=mu and E(Y)=v

    Well I found that formula on wikipedia, but it doesn't make sense to me because if E(X)=mu then doesn't E(X-mu) equal zero?

    Well thats my main problem, I dont know how to use the formula.

    3. The attempt at a solution

    I've worked out the distribution table to see if that would help

    . . . . y
    . . . -1___0___1
    . -1 0, 1/8, 1/4 (X=-1) =3/8
    x 0 1/8, 1/4, 3/8 (X=0) =3/4
    . .1 1/4, 3/8, 1/2 (X=1)=1/18
    (Y=-1) = 3/8 (Y=0)=3/4 (Y=1) = 1/1/8

    Sorry its not in LaTex I couldn't remember how to start the code.

    I've also worked out that the var(Y) = 1 if that is any help? I have an exam on this very soon and I really want to be able to get a hold of all the main concepts... thanks for any information
  2. jcsd
  3. May 25, 2008 #2
    [tex]cov(X,Y) = E[(X-\mu)(Y-\nu)] = E(X Y) - \mu \nu[/tex]

    You are right to say that [tex]E(X) = \mu, E(Y) = \nu[/tex] which is why the cross terms are gone, but...

    since the pdf is not separable the variables are correlated and [tex]E(XY) \neq E(X) E(Y)[/tex] and your expression is non-zero.

    You must evaluate the expectation values by integration. What is your domain? I ask that because it can't be over the whole space, because your pdf would be non-normalizable.
  4. May 25, 2008 #3
    The expectation value for a random variable, say for example [tex]X^3 Y^2[/tex], is given by

    [tex]E(X^3 Y^2) = \int dx dy f(x,y) x^3 y^2[/tex] if your distribution is already normalized else it's given by

    [tex]E(X^3 Y^2) = \frac{\int dx dy f(x,y) x^3 y^2}{\int dx dy f(x,y)}[/tex]
  5. May 26, 2008 #4
    The domain is -1 < x < 1 and -1 < y < 1
  6. May 26, 2008 #5


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    Out of curiosity, why would she need to normalize it? From what I understand normalizing would be done to find the correlation matrix.
    Last edited: May 26, 2008
  7. May 26, 2008 #6
    Well the expectation value/mean/average wouldn't be uniquely defined otherwise. It has nothing to do with the correlation matrix, it has to do with the fact that [itex]E(1) = \int f dA = P(\textup{entire domain}) = 1[/itex]. See, it's just one step from an axiom of probability theory.
  8. May 26, 2008 #7


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    Ah of course. I usually check that the pdf integrates to 1 over the domain (general case with classroom problems) and it slipped my mind that this may not be a case like that.
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