On consecutive evenings a researcher sets the lights in a restaurant at 40, 60, and 100 watts, and has either music or no music, until all possible combinations are met. That is, on Monday the lights are at 40 watts, with no music (NM), on Tuesday the lights are at 40 watts with music (M), Wednesday 60 watts with NM, Thursday 60 watts with M, Friday 100 NM, and Saturday 100 M (hint: draw a matrix). She uses the same staff each night. At the end of each evening, she determines the total amount made in tips. What formal question is this experiment asking