Math Help Forum

Math Help Forum Feed Site Feed

Go Back   Math Help Forum > University Math Help > Advanced Probability and Statistics
Reply
 
Thread Tools Display Modes
  #1  
Old October 20th, 2009, 11:47 AM
Junior Member
 
Join Date: Sep 2008
Posts: 58
Country:
Thanks: 28
Thanked 0 Times in 0 Posts
Oblivionwarrior is on a distinguished road
Default Covariance and Correlation(continuous random variables)

Determine the value of c and the covariance and correlation for the joint probability density function f(x, y) = c for 0< x < 5, 0 < y and x - 1 < y < x + 1.

I am confused on how to set-up for the bounds for the integration. It looks like you have to split the region into 2 sections because of the 0 < y, but I am not sure. Thanks for any help.

Last edited by Oblivionwarrior; October 20th, 2009 at 11:58 AM.
Reply With Quote
Advertisement
 
  #2  
Old October 20th, 2009, 06:53 PM
mr fantastic's Avatar
Flow Master

 
Join Date: Dec 2007
Location: Zeitgeist
Posts: 13,546
Country:
Thanks: 2,997
Thanked 5,264 Times in 4,613 Posts
mr fantastic has a reputation beyond reputemr fantastic has a reputation beyond reputemr fantastic has a reputation beyond reputemr fantastic has a reputation beyond reputemr fantastic has a reputation beyond reputemr fantastic has a reputation beyond reputemr fantastic has a reputation beyond reputemr fantastic has a reputation beyond reputemr fantastic has a reputation beyond reputemr fantastic has a reputation beyond reputemr fantastic has a reputation beyond repute
Default

Quote:
Originally Posted by Oblivionwarrior View Post
Determine the value of c and the covariance and correlation for the joint probability density function f(x, y) = c for 0< x < 5, 0 < y and x - 1 < y < x + 1.

I am confused on how to set-up for the bounds for the integration. It looks like you have to split the region into 2 sections because of the 0 < y, but I am not sure. Thanks for any help.
Can you draw the region on a graph. It's then very clear that the region needs to be split into two sections (which two sections will depend on what order of integration you choose. Personally I'd integrate first wrt y).
__________________
There are two things you should never try to prove: the impossible and the obvious.

The greater danger for most of us lies not in setting our aim too high and falling short; but in setting our aim too low and achieving our mark. (Michelangelo Buonarroti)

  • To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.

  • To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.

  • To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.

  • To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.

  • To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
Reply With Quote
  #3  
Old October 20th, 2009, 10:01 PM
matheagle's Avatar
MHF Contributor
 
Join Date: Feb 2009
Posts: 1,957
Country:
Thanks: 127
Thanked 813 Times in 730 Posts
matheagle is a splendid one to beholdmatheagle is a splendid one to beholdmatheagle is a splendid one to beholdmatheagle is a splendid one to beholdmatheagle is a splendid one to beholdmatheagle is a splendid one to beholdmatheagle is a splendid one to beholdmatheagle is a splendid one to behold
Send a message via Yahoo to matheagle
Default

Either way you need two regions to integrate.
But to find c you just need the area and calculus is not needed for that.
You just find the area of the larger triangle and subtract off the area of the smaller one.
Reply With Quote
  #4  
Old October 21st, 2009, 08:59 AM
Junior Member
 
Join Date: Sep 2008
Posts: 58
Country:
Thanks: 28
Thanked 0 Times in 0 Posts
Oblivionwarrior is on a distinguished road
Default

Ok. I found C to be 2/19. I integrated over the 2 regions. For the smaller region I got 3/2C, and for the larger region I got 8C. So 3/2C + 8C = 1, therefore C = 1/9.5 or 2/19.

To find the covariance would I use the formula E(XY) - E(X)E(Y)? To find E(XY) would I integrate again over the regions but this time have xy inside the integral? Would the C be 2/19 for both sides or would I need to split it up for both sides?
Reply With Quote
  #5  
Old October 22nd, 2009, 01:32 AM
Member
 
Join Date: Jan 2009
Posts: 102
Country:
Thanks: 4
Thanked 25 Times in 23 Posts
The Second Solution is on a distinguished road
Default

Quote:
Originally Posted by Oblivionwarrior View Post
Ok. I found C to be 2/19. I integrated over the 2 regions. For the smaller region I got 3/2C, and for the larger region I got 8C. So 3/2C + 8C = 1, therefore C = 1/9.5 or 2/19.

[snip]
As has already been suggested, since the joint pdf is constant you can use simple geometry:

You require c(area of trapezium - area of triangle) = 1.
Area of trapezium = 5(1 + 6)/2 = 35/2.
Area of triangle = (4)(4)/2 = 8.
c(35/2 - 8) = 1 => c = 2/19 so yes you're correct.

Quote:
Originally Posted by Oblivionwarrior View Post
[snip]
To find the covariance would I use the formula E(XY) - E(X)E(Y)? To find E(XY) would I integrate again over the regions but this time have xy inside the integral? Would the C be 2/19 for both sides or would I need to split it up for both sides?
Yes. Yes. Yes it would - c = 2/19 is the pdf so why on Earth would you split it up??
Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off
Forum Jump


All times are GMT -7. The time now is 05:58 AM.


Powered by vBulletin® Version 3.7.3
Copyright ©2000 - 2010, Jelsoft Enterprises Ltd.
SEO by vBSEO 3.2.0 ©2008, Crawlability, Inc.
©2005 - 2009 Math Help Forum


Math Help Forum is a community of maths forums with an emphasis on maths help in all levels of mathematics.
Register to post your math questions or just hang out and try some of our math games or visit the arcade.