Question: Lets return to the height-weight example on page 53 and recall what happened when we added a nonsensical variable that measured the students campus post
Let’s return to the height-weight example on page 53 and recall what happened when we added a nonsensical variable that measured the student’s campus post office box number (MAIL) to the equation. The estimated equation changed from:
To:
a. The estimated coefficient of HEIGHT changed when we added MAIL to the equation. Does that make sense? Why?
b. In theory, someone’s weight has nothing to do with their campus mail box number, yet R2 went up from .74 to .75 when MAIL was added to the equation! How is it possible that adding a nonsensical variable to an equation can increase R2?
c. Adding the nonsensical variable to the equation decreased R2 from .73 to .72. Explain how it’s possible that R2 can go down at the same time that R2 goes up.
d. If a person’s campus mail box number truly is unrelated to their weight, shouldn’t the estimated coefficient of that variable equal exactly 0.00? How is it possible for a nonsensical variable to get a nonzero estimated coefficient?
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