An ad for Crest Whitestrips Premium claims that the strips will whiten teeth in 7 days and the results will last for 12 months. A researcher who wishes to test this claim studies 20 sets of identical twins. Within each set of twins, one is randomly selected to use Crest Whitestrips Premium in addition to regular brushing and ﬂossing, while the other just brushes and ﬂosses. Whiteness of teeth is measured at the beginning of the study, after 7 days, and every month thereafter for 12 months.
(a) What type of experimental design is this?
(b) What is the response variable in this experiment?
(c) What are the treatments?
(d) What are other factors (controlled or uncontrolled) that
could affect the response variable?
(e) What might be an advantage of using identical twins as
subjects in this experiment?