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October 5th, 2011, 07:14 PM  #1 
Senior Member Joined: Mar 2011 Posts: 105 Thanks: 0  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?? 
October 6th, 2011, 12:48 PM  #2 
Global Moderator Joined: May 2007 Posts: 6,730 Thanks: 689  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. 
October 6th, 2011, 02:58 PM  #3  
Senior Member Joined: Mar 2011 Posts: 105 Thanks: 0  Re: Statistics Quote:
 
October 7th, 2011, 01:21 PM  #4 
Global Moderator Joined: May 2007 Posts: 6,730 Thanks: 689  Re: Statistics
No correlation means correlation = 0

October 9th, 2011, 04:53 PM  #5  
Senior Member Joined: Mar 2011 Posts: 105 Thanks: 0  Re: Statistics Quote:
 
October 9th, 2011, 04:57 PM  #6 
Senior Member Joined: Mar 2011 Posts: 105 Thanks: 0  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 
October 10th, 2011, 01:27 PM  #7 
Global Moderator Joined: May 2007 Posts: 6,730 Thanks: 689  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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