Why is it difficult to measure the effect of food away from home on diet quality?

To answer the question look for clues in the second sentence in addition to the word "why." The answer lies in "many unobserved factors" and the more specific "such as food preferences and time constraints." The correct answer is supported by the following: "many unobserved factors, such as food preferences and time constraints, influence not just our choice of where to eat but also the nutritional quality of what we eat."