a.-e. Repeat Problem 8, parts (a) through (e), after centering the predictor (age).
f. Compare the results obtained here to those obtained in Problem 8.
Problem 8
This problem uses the data presented in the table for Problem 12 in Chapter 5. Use α = .05.
a. Use a computer program to fit a natural polynomial cubic model for predicting vocabulary size as a function of age in years. Provide estimated regression coefficients.
b. Using variables-added-in-order tests, determine the best model.
c. Report variables-added-last tests. Explain any differences between your results here and those you obtained in part (b).
d. Report appropriate collinearity diagnostics for the model, and evaluate them. Include predictor correlations.
e. Plot jackknife (or studentized) residuals against predicted values for the best model based on your results in part (b). Provide a frequency histogram or schematic plot of the residuals, and comment on it.