EXAMPLE 8 Can low-fat food labels lead to obesity?

What are the effects of low-fat food labels on food consumption? Do people eat more of a snack food when the food is labeled as low fat? The answer may depend both on whether the snack food is labeled low fat and whether the label includes serving-size information. An experiment investigated this question using university staff, graduate students, and undergraduate students at a large university as subjects. Over 10 late-afternoon sessions, all subjects viewed episodes of a 60-minute, made-for-television program in a theater on campus and were asked to rate the episodes. They were also told that because it was late in the afternoon, they would be given a cold 24-ounce bottle of water and a bag of granola from a respected campus restaurant called The Spice Box. They were told to enjoy as much or as little of it as they wanted. Each participant received 640 calories (160 grams) of granola in ziplock bags that were labeled with an attractive 3.25- ×- 4-inch color label. Depending on the condition randomly assigned to the subjects, the bags were labeled either “Regular Rocky Mountain Granola” or “Low-Fat Rocky Mountain Granola.” Below this, the label indicated “Contains 1 Serving” or “Contains 2 Servings,” or it provided no serving-size information. As participants left the theater, they were asked how many serving sizes they believed their package contained. Out of sight of the participants, the researchers also weighed each granola bag. Participants’ statements about serving size and the actual weights of the granola bags are the response variables.

This experiment has two explanatory variables: fat content, with two levels, and serving size, with three levels. The six combinations of one level of each variable form six treatments. Figure 6.1 shows the layout of the treatments.