Masha weighed 7.5 lbs when she was born. Then her weight increased according to the table.

(a) Construct a time plot of Masha’s weight. What regression model seems appropriate for these data?

(b) Fit a quadratic model y = β0 + β1x + β2x2 + ε. Why does the estimate of β2 appear negative?

(c) What additional portion of the total variation is explained by the quadratic term? Would you keep it in the model?

(d) Predict Masha’s weight at 24 months using the quadratic model and using the linear model. Which prediction is more reasonable?

(e) Can one test significance of the quadratic term by an F-test? Why or why not?

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