Households that do not have a microwave oven use less energy per year, on average, than do households that have a microwave oven. Households whose microwave oven does not have a built-in clock are no more likely to have a separate electric clock plugged in than households whose microwave oven has one. All households that have a microwave oven also have either a gas oven or a conventional electric oven. There are more households that have a microwave oven with a built-in clock than there are households that have a microwave oven without a built-in clock.
Most household appliances use electricity only when in use. Many microwave ovens, however, have built-in clocks and so use some electricity even when they are not in use. The clocks each consume about 45 kilowatt-hours per year. Ttherefore, households whose microwave oven has no built-in clock use 45 kilowatt-hours per year less, on average, than do comparable households whose microwave oven is otherwise similar but has a built-in clock.
Which of the following is an assumption on which the argument depends?