A government study is undertaken with the aim of determining the benefits of a new driver training scheme and whether it has a different outcome on over-25 years old learner drivers than for under-25 years old learner drivers. The training program is undertaken for a group of 80 learner drivers, half of which are under 25 years old, half are over 25 years old. Then within each group of 40, half are randomly selected to participate in the new training program. The results are recorded and compared. This scenario is best described as an example of: