Physical Therapy Therapists at a physical therapy clinic treat many athletes for knee pain. The clinic has used traditional land-based therapy for many years. The therapists ask for volunteers to try aquatic therapy as an alternative. Volunteers are then randomly assigned to either the traditional land-based therapy group or the aquatic therapy group. After a month of treatment, athletes in both groups are asked to rate their knee pain on a scale from 1 (no pain) to 10 (constant terrible pain). The aquatic therapy group reports more pain than the traditional group. The therapists decide to stop using aquatic therapy. (a) What is the response variable? (b) What groups are being compared? (c) Explain how selection bias is likely to play a role in why those in the aquatic group report more knee pain than those in the traditional group.