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  #141  
Old 07-11-2008, 03:38 PM
Actuarialsuck Actuarialsuck is offline
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Assuming you didn't know about it, though you probably should as it will certainly pop up in 4/C, here's a way to do a problem in an equivalent way.

Assuming you know that V(X) = E(V(X|Y)) + V(E(X|Y)) let's calculate the 2 pieces we are summing



since we are just treating V(X|Y) as a R.V. and finding the expected value by multiplying outcomes by prob. of those outcomes



You should know this simply because you should know that V(Blah) = E(Blah^2) - [E(Blah)]^2, well here Blah is E(X|Y).

Now,



and



therefore



and finally

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  #142  
Old 07-12-2008, 01:05 AM
gametime50 gametime50 is offline
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Default May 12/08 question

Could you remind me again what that shortcut is? I have not seen or heard of it i just know the variance of the bernoulli being pq?
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  #143  
Old 07-12-2008, 09:14 PM
Actuarialsuck Actuarialsuck is offline
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Quote:
Originally Posted by gametime50 View Post
Could you remind me again what that shortcut is? I have not seen or heard of it i just know the variance of the bernoulli being pq?
Suppose you have a R.V. X that has outcome x with prob. p(x) and outcome y with prob. p(y).

Then

You can derive this formula just by computing moments and knowing your algebra

Spoiler:




and



therefore since




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  #144  
Old 07-14-2008, 04:41 PM
gametime50 gametime50 is offline
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Default May 12/08 question

Thank you for the response. I wander how often that will show up for exam P? Im just starting the testing process so I havent seen any of the stuff beyond Exam P really. I think you meant to have a couple - signs in there instead of + but i got what you meant. Thanks again.
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  #145  
Old 07-14-2008, 04:43 PM
Actuarialsuck Actuarialsuck is offline
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Where?
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  #146  
Old 07-15-2008, 12:37 AM
gametime50 gametime50 is offline
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Default May 12/08 question

in the blah parts
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  #147  
Old 07-15-2008, 12:41 AM
Actuarialsuck Actuarialsuck is offline
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Quote:
Originally Posted by gametime50 View Post
in the blah parts
What do you mean? Point to actual places in the derivation.
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  #148  
Old 07-15-2008, 07:36 PM
gametime50 gametime50 is offline
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Default may 12/08 question

No you did the problem fine i think you made a typo when you had the v(blah)= E(blah^2) + instead of - (E(blah))^2
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  #149  
Old 07-18-2008, 09:35 AM
sam_broverman sam_broverman is offline
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Actuarialscuk,

Thanks for responding to the question. I was
out of internet contact for a little while.

Sam Broverman
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  #150  
Old 07-18-2008, 10:57 AM
Actuarialsuck Actuarialsuck is offline
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Quote:
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No you did the problem fine i think you made a typo when you had the v(blah)= E(blah^2) + instead of - (E(blah))^2
I finally saw what you were talking when I realized I actually used the word "blah". Thanks, I corrected it.
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