A weight-management researcher was interested in whether the size of a person's breakfast could deter overall food consumption throughout the rest of the day. He creates two breakfast groups, a 350-calorie breakfast and a 750-calorie breakfast; assigns six participants to each group; and tracks the total calories participants eat in one day. Because of the detailed attention needed to accurately interview participants about their eating, he works with the high-calorie group and has his assistant interview the low-calorie group. What is the independent variable in this study