Callahan (2009) demonstrated that Tai Chi can significantly reduce symptoms for individuals with arthritis. Participants were 18 years old or older with doctordiagnosed arthritis. Self-reports of pain and stiffness were measured at the beginning of an eight-week Tai Chi course and again at the end. Suppose that the data produced an average decrease in pain and stiffness of M_D = 8.5 points with a standard deviation of 21.5 for a sample of n = 40 participants.

a. Use a two-tailed test with α = .05 to determine whether the Tai Chi had a significant effect on pain and stiffness.

b. Compute Cohen’s d to measure the size of the treatment effect.