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THE BOOK--Playing The Percentages In Baseball

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Tuesday, December 04, 2007

Giving the finger to correlation coefficient

By Tangotiger, 10:06 AM

I get mighty p.o.ed when talk comes to correlation coefficient (r), and how the r-squared “explains” something.  What it’s really saying is that, given that size of sample, the variance of the parameter you are studying explains a certain percentage of the total variance (including the variance from luck which is completely based on the size of the sample).  You can, in effect, control your r-squared to your liking.  Wanna prove clutch exists?  Use a sample size of 5000 PA.  Wanna prove clutch doesn’t exist?  Use a sample size of 50.

Anyway, while I get mighty p.o.ed, Phil takes it to another level.  Here are his archives on the matter.

If you see a researcher (baseball or otherwise) give a correlation coefficient, without giving you a regression equation (like Phil does), or a correlation coefficient equation (like r=PA/[PA+200]), give him the finger.

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