Astudy attempts to compare two sunscreens. each of 50 subjects with varying skin complexions will use both sunscreens—screen a on one side of his or her body and screen b on the other side. for each subject, a coin is tossed in order to determine which side receives screen a and which receives screen b. researchers measure the amount of ultraviolet light exposure over both treated areas for each subject. this is an example of:
a. the placebo effect. b. a stratified analysis. c. a double-blind observational study. d. a matched pairs experiment.