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October 5th, 2011, 07:14 PM   #1
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Statistics

For their first exam, students in an AP Statistics class studied an average of 4 hours with a standard deviation of 1 hour. Almost everyone did poorly on the exam, and so for the second exam every student studied 10 hours. What is the correlation between the numbers of hours students studied for each exam??

Solving the problem, I put some pairs like (3,10) (4,10 ) (2,10) (5,10)
and of course, the calculator didn't give me the correlation because the standard deviation of y values is 0, and it is a denominator in the correlation formula

but my professor said the answer is r = 0..Can someone explain me, please??
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October 6th, 2011, 12:48 PM   #2
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Re: Statistics

There is no correlation because the number of hours for the second time is independent of the number of hours for the first time.
In general independence implies no correlation.
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October 6th, 2011, 02:58 PM   #3
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Re: Statistics

Quote:
Originally Posted by mathman
There is no correlation because the number of hours for the second time is independent of the number of hours for the first time.
In general independence implies no correlation.
so, is it 0 or can't be found?
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October 7th, 2011, 01:21 PM   #4
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Re: Statistics

No correlation means correlation = 0
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October 9th, 2011, 04:53 PM   #5
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Re: Statistics

Quote:
Originally Posted by mathman
No correlation means correlation = 0
thank you
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October 9th, 2011, 04:57 PM   #6
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Re: Statistics

Can someone help me about it?

If there are 5 histograms

one is right skewed
one is left skewed
one is uniform
one is unimodal symetric
one is bimodal symetric

which one have smallest standard deviation
which one have largest standard deviation

those scales are about the same
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October 10th, 2011, 01:27 PM   #7
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Re: Statistics

There is not enough information. Either of the two skewed or the unimodal could be the correct answer, depending on how fat they are.
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