Masha weighed 7.5 lbs when she was born. Then her weight increased according to the table.
(a) Construct a time plot of Mashaâ€™s weight. What regression model seems appropriate for these data?
(b) Fit a quadratic model y = Î²0 + Î²1x + Î²2x2 + Îµ. Why does the estimate of Î²2 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 Mashaâ€™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?