Investigation 12: Smoking and lung capacity (assigned Fri Feb 26, due Wed Mar 3)

You may work with one other person on this assignment, handing in one report with both names. Word-processed reports are much preferred to hand-written ones. Please copy/paste relevant Minitab output into a Word file as appropriate.

Forced expiratory volume (FEV), the amount of air an individual can exhale in the first second of forceful breath (in liters), was measured for a group of 654 people, along with whether or not they smoke and several other variables including age. The data are in the Minitab worksheet FEV252.mtw.

a) Produce (and submit) comparative boxplots of FEV, comparing smokers and nonsmokers. Comment on what these boxplots reveal.

b) Fit a (simple) regression model for predicting FEV based only on the (coded) smoking variable. Report the regression equation and R² value. Also determine the predicted FEV for a smoker and for a nonsmoker.

c) For the regression model in b), report the t-test statistic and p-value. What do you conclude about whether, and if so how, smoking is related to FEV?

d) Now fit a (multiple) regression model for predicting FEV based on age, smoking, and the interaction between age and smoking. Report the regression equation and R² value. Determine separate linear equations for predicting FEV based on age, separately for a smoker and for a nonsmoker.

e) For the regression model in d), report the three t-test statistic and p-values. Which of these variables (if any) are significant predictors of FEV at the .05 level?

f) Produce a scatterplot of FEV vs. age, using different symbols for smokers and nonsmokers, with separate regression lines drawn.

g) Predict the FEV for a 16-year-old smoker and for a 16-year-old nonsmoker, both with point estimates and with 95% prediction intervals.

h) Summarize what your analyses reveal about the relationship among smoking, age, and FEV.