A study of blood flow involved 27 healthy people aged 18 to 72. Each subject consumed a cocoa beverage containing 900 milligrams of flavonols daily for 5 days. Using a finger cuff, blood flow was measured on the first and fifth days of the study. After 5 days, researchers measured what they called “significant improvement” in blood flow and the function of the cells that line the blood vessels. What flaw in the design of this experiment makes it impossible to say if the cocoa really caused the improved blood flow? Explain your answer.