A pediatrician is interested in the long term effects of an experimental medical treatment designed to improve joint flexibility in children affected with arthritis. A measure of joint flexibility is taken on 12 children randomly selected for the study. After using the experimental treatment for 12 months, a second measure of joint flexibility is taken. The mean increase in flexibility is 4.6 units with a standard deviation of 2.1 units. Construct a 90% confidence interval for the true mean increase in joint flexibility. (assume that the difference scores are approximately normally distributed.)