Does playing action video games provide more than just entertainment? The authors of the paper "Action- Video- Game Experience Alters the Spatial Resolution of Vision" (Psychological Science [2007]: 88- 94) concluded that spatial resolution, an important aspect of vision, is improved by playing action video games. They based this conclusion on data from an experiment in which 32 volunteers who had not played action video games were "equally and randomly divided between the experimental and control groups." Subjects in each group played a video game for 30 hours over a period of 6 weeks. Those in the experimental group played Unreal Tournament 2004, an action video game. Those in the control group played Tetris, a game that does not require the user to process multiple objects at once. Explain why it was important for the researchers to randomly assign the subjects to the two groups.