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# Section2.2posted - STOR155,Section2 Tuesday,January26,2010...

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STOR 155, Section 2 Tuesday, January 26, 2010 Section 2.2

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Figure 2.9 Introduction to the Practice of Statistics, Sixth Edition © 2009 W.H. Freeman and Company Section 2.2:  Correlation Which association is stronger? The correlation r between two quantitative variables is a number between −1 and +1 that tells us the direction and strength of the association between them.
Correlation:  formula  (optional!) Individual  i x i y i (x x)/s x (y i  -  y)/s y product 1 3 9 - 0.97 + 0.68 - 0.667 2 4 10 - 0.65 + 1.03 - 0.667 3 6 5 0 - 0.69 0 4 6 3 0 - 1.37 0 5 11 8 + 1.62 + 0.34 +0.556 x = 6 y = 7  Sum =  –0.778 s x  = 3.1 s y  = 2.9 r =  –0.778/4  0.19 Example:

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Correlation doesn’t work with  nonlinear  associations

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Properties of Correlation The  sign  of  indicates the  direction  of the  association (assuming it is linear).
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Section2.2posted - STOR155,Section2 Tuesday,January26,2010...

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