linear correlation

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chiu

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Suppose i have a table of (x)s and their f(x), x representing grades at some exam. n=100.
and i'm told that the correlation between the grades and the number of hours studied stands on r=0.85.
Furthermore, 1h of study costs 30$ and a one off exam signup costs 40$.
I have to find out whats the r between the grades and the costs.
Is it correct to say y=30x+40 where y represents the cost? however x stands for hours of study here, doesnt it? I used the list of grades to express the costs using this linear transformation but i dont think it's correct. I dont know how to use the 'r=0.85'...

Thanks guys
 
chiu said:
Suppose i have a table of (x)s and their f(x), x representing grades at some exam. n=100.
and i'm told that the correlation between the grades and the number of hours studied stands on r=0.85.
Furthermore, 1h of study costs 30$ and a one off exam signup costs 40$.
I have to find out whats the r between the grades and the costs.
Is it correct to say y=30x+40 where y represents the cost? however x stands for hours of study here, doesnt it? I used the list of grades to express the costs using this linear transformation but i dont think it's correct. I dont know how to use the 'r=0.85'...

Thanks guys
Click to expand...
Don't use "x" for two different things - that can only lead to confusion. If x = grade, then let t = hours of study.
\(\displaystyle y = ($30/hr)\ t + $40\)
is an exact linear equation for cost as a function of time.

There is a linear model that expresses x as a function of t, and you are told that the correlation is 85%. That means that 85% of the variation of x (about its mean) can be explained as being proportional to the difference of t from its mean. What happens when you consider the linear relation between t and y?
 
I guess i can express t as ((+)y-40)/30
grade remains (+)x
then rx,y= (1*1/|1*1|)*0.85 = 0.85


either this or I'm completely out of my depth here
 
chiu said:
I guess i can express t as ((+)y-40)/30
grade remains (+)x
then rx,y= (1*1/|1*1|)*0.85 = 0.85


either this or I'm completely out of my depth here
Click to expand...
I believe that is correct. If we are wrong, there are other tutors here who will set us straight.
 
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