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 MD 5 8.5 points with a standard deviation of 21.5 for a sample of n 5 40 participants.

a. Use a two-tailed test with a 5 .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.