Julie works at a large gym. She wants to do a study of the effect on self-esteem and mood of having a personal trainer. She collects a list of members at her gym who would like to have a personal trainer. She assigns half of them to have a trainer right away and half of them to wait several months before being assigned a trainer. She measures self-esteem and mood in all of the participants over the next several months, when only one of the groups would have a trainer. Which advantages does Julie's design have over selecting a group of people at the gym who were already using personal trainers and a group who was not using trainers