An article in Nature describes an experiment to investigate the effect on consuming chocolate on cardiovascular health (“Plasma Antioxidants from Chocolate,” 2003, Vol. 424,). The experiment consisted of using three different types of chocolates: 100 g of dark chocolate, 100 g of dark chocolate with 200 ml of full-fat milk, and 200 g of milk chocolate. Twelve subjects were used, seven women and five men with an average age range of 32.2 ±1 years, an average weight of 65.8 ± 3.1 kg, and body-mass index of 21.9 ± 0.4 kg m−2. On different days, a subject consumed one of the chocolate-factor levels, and one hour later total antioxidant capacity of that person’s blood plasma was measured in an assay. Data similar to those summarized in the article follow.

(a) Construct comparative box plots and study the data. What visual impression do you have from examining these plots?

(b) Analyze the experimental data using an ANOVA. If α = 0.05, what conclusions would you draw? What would you conclude if α = 0.01?

(c) Is there evidence that the dark chocolate increases the mean antioxidant capacity of the subjects’ blood plasma?

(d) Analyze the residuals from this experiment.