Chapter 2.3, Problem 36E

The following is from an article titled “After the Workout, Got Chocolate Milk?” that appeared in the Chicago Tribune (January 18, 2005):

Researchers at Indiana University at Bloomington have found that chocolate milk effectively helps athletes recover from an intense workout. They had nine cyclists bike, rest four hours, then bike again, three separate times. After each workout, the cyclists downed chocolate milk or energy drinks Gatorade or Endurox (two to three glasses per hour); then, in the second workout of each set, they cycled to exhaustion. When they drank chocolate milk，the amount of time they could cycle until they were exhausted was similar to when they drank Gatorade and longer than when they drank Endurox.

The article is not explicit about this, but in order for this to have been a well-designed experiment, it must have incorporated random assignment. Briefly explain where the researcher would have needed to use random assignment in order for the stated conclusion to be valid.