A nutritional researcher was interested in whether the size of breakfast could decrease overall food consumption throughout the rest of the day. He creates two breakfast groups, a 350-calorie breakfast and a 700-calorie breakfast, assigns eight participants to each group, and tracks their total calories eaten 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 confounding variable in this study