Read pages 181-189 from your text (up to “More About Assumptions/Conditions”)

From pages 199-202, do exercises 11, 12, 31, 37

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Read pages 181-189 from your text (up to “More About Assumptions/Conditions”)

From pages 199-202, do exercises 11, 12, 31, 37

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Why the mean of the residual =0?

Because some of the residuals will be positive and some will be negative. And just like with the calculation of the standard deviation, all of the positive residuals will exactly cancel out the negatives, resulting in an average of zero. This is why we need to square the residuals when considering the line of best fit.

why would they “exactly” cancel out?

I sketched out a proof. The average of residuals is zero because the sum of all y-yhat is zero. https://mrkirkmath.files.wordpress.com/2017/10/img_20171012_135025.jpg

I got it. Thank you so much.