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# Method of calculating p value for Pearson's correlation coefficient

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### #1 Curtis

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Posted 28 April 2013 - 09:35 PM

I'm not very professional in calculating Pearson's correlation coefficient (r), and I see people use the following formula to calculate the p value for r:

p=r/Sqrt(r^2)/(N—2)

where N is the number of variants.

why don't they just use student's t test? I usually use Excel and chose TTEST function for this matter. It's so easy.

### #2 bob1

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Posted 29 April 2013 - 02:22 PM

Pearson's is used for determining if there is a correlation between variables, a t-test measures the difference between two populations, so they look at different things.

### #3 DRT

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Posted 29 April 2013 - 03:37 PM

The formula you have given looks like it should be t=r/Sqrt((1-r^2)/(N—2)) the t statistic for testing if the correlation coefficient is significant, it can then be converted to a p value using the T.DIST function in Excel.

### #4 Curtis

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Posted 29 April 2013 - 09:21 PM

Pearson's is used for determining if there is a correlation between variables, a t-test measures the difference between two populations, so they look at different things.

I always use TTEST function and I have no problem. But the above equation gives me different p value which is not similar to the TTEST value. I am still not sure why people use this equation to calculate the p value of those variables. I hope you understand what I' trying to say. I know r is correlation coefficient. by why can't they just use the same TTEST, instead of this equation. it shouldn't be used?

### #5 Curtis

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Posted 29 April 2013 - 09:35 PM

The formula you have given looks like it should be t=r/Sqrt((1-r^2)/(N—2)) the t statistic for testing if the correlation coefficient is significant, it can then be converted to a p value using the T.DIST function in Excel.

I checked TDIST. don't know how to fill those gaps.

### #6 DRT

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Posted 30 April 2013 - 01:01 PM

The formula you have given looks like it should be t=r/Sqrt((1-r^2)/(N—2)) the t statistic for testing if the correlation coefficient is significant, it can then be converted to a p value using the T.DIST function in Excel.

I checked TDIST. don't know how to fill those gaps.

I found this link with the instructions for getting the obtaining a p value from the correlation coefficient.
http://www.ehow.com/...excel-2007.html

You may also like to install the ‘data analysis’ add-in for excel (under File-Options-Add-Ins) but make sure you come to grips with the different sorts of t-tests first