12.64 Stem cells In the article, “Variation in cancer risk among tissues can be explained by the number of stem cell divisions” (Tomasetti and Vogelstein, Science, vol.

47, 2015), the authors stated: “A linear correlation equal to 0.804 suggests that 65% of the differences in cancer risk among different tissues can be explained by the total number of stem cell divisions in those tissues.”

a. How did they get 65%?

b. By how many percentage points does the overall prediction error (as measured by the sum of squares) decrease when using the prediction equation from a linear regression model to predict cancer risk rather than using the sample mean of the observed risks to predict it?