A researcher did an experiment in which he had one group of people drink a liter of coffee in an hour and a control group of people drink a liter of water within the same amount of time. He then measured their urine output over the course of the next two hours. Both groups produced an average of 250 ml of urine per hour over that time period. He concluded that the coffee itself had no effect on urine output. Given his results, what else should he measure to identify the effects of coffee drinking?