Bart wants to know whether skateboarding decreases reaction time (makes you react faster). He recruits 50 children who haven't skateboarded before. He randomly assigns about half of them to the "skateboarding" group, and the other half to the control group. The "skateboarding" group has skateboarding lessons after school for half an hour every day for two weeks. The control group meets for half an hour every day for two weeks after school and does homework. After the two weeks are up, Bart measures the reaction time of the members of both groups. What type of design is this