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The national farming association has data on the amount of pesticide used per acre and the percentage of fruit that has been contaminated by insects per acre, for $117$ farms. These two variables have a correlation coefficient $r=-0.8$. For a farm that uses an amount of pesticide that is $2$ standard deviation above the mean amount of pesticide used, we would predict the percentage of contaminated fruit will be $1.6$ SD below the mean.

Can anyone explain how $1.6$ below the mean is calcuated to me? I can only think of sum of $\cfrac{z_x z_y}{n-1}= r.$

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$-1.6=2 \times (-0.8)$. –  hhsaffar Dec 12 '13 at 15:30
    
what formula is that? –  natsu Dec 12 '13 at 15:31
    
First, this is not an exact formula, this is a prediction. There is usually some error. You may find this helpful onlinestatbook.com/2/regression/intro.html –  hhsaffar Dec 12 '13 at 15:37

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