13.21 The effects of proximity and visibility on food intake. A study investigated the influence that proximity and visibility of food have on food intake.¹⁴ A total of 40 secretaries from the University of Illinois participated in the study. A candy dish full of individually wrapped chocolates was placed either at the desk of the participant or at a location 2 meters from the participant. The candy dish was either a clear (candy visible) or opaque (candy not visible) covered bowl. After a week, the researchers noted not only the number of candies consumed per day, but also the self-reported number of candies consumed by each participant. The following table summarizes the mean difference between these two values (reported minus actual):

	Visibility
Proximity	Clear	Opaque
Proximate	−1.2	−0.8
Less proximate	0.5	0.4

(a) Make a plot of the means and describe the patterns that you see. Does the plot suggest an interaction between visibility and proximity?
(b) This study actually took four weeks, with each participant being observed at each treatment combination in a random order. Explain why a repeated-measures design like this may be beneficial.