In studying the effects of Tai Chi on arthritis pain, Callahan (2009) selected a large sample of individuals with doctor-diagnosed arthritis. Half of the participants immediately began a Tai Chi course and the other half waited 8 weeks before beginning the program. At the end of 8 weeks, those who had experienced Tai Chi had less arthritis pain those who had not participated in the program. Is this an example of an experimental or a non-experimental study? (.5)