# Expected Value

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• Sep 11th 2008, 05:33 AM
janvdl
Expected Value
Quote:

Let: X ~ Poisson( $\lambda$ )

Find: $E[X(X-1)]$

There is a certain method to go about doing this. I remember the start and finish, but not the intermediate steps.

Quote:

$E[X^2 - X]$

$= E[X^2] - E[X]$

... ... ... ...

$= \lambda + \lambda ^2 - \lambda$

$= \lambda ^2$

Or something like it. It had to do with the fact that E[X] in this case = $\lambda$
• Sep 11th 2008, 05:42 AM
mr fantastic
Quote:

Originally Posted by janvdl
There is a certain method to go about doing this. I remember the start and finish, but not the intermediate steps.

Or something like it. It had to do with the fact that E[X] in this case = $\lambda$

Hey there Jan!

Try reading this thread: http://www.mathhelpforum.com/math-he...-one-more.html

Then let me know if you're still stuck.
• Sep 11th 2008, 05:48 AM
janvdl
Quote:

Originally Posted by mr fantastic
Hey there Jan!

Try reading this thread: http://www.mathhelpforum.com/math-he...-one-more.html

Then let me know if you're still stuck.

Thanks Mr F, I have some sort of idea now (Nod)