In a study15 of the effect of the compound tomoxetine as a treatment for adult attention deficit hyperactivity disorder (ADHD), the 21 subjects had an ADHD rating scale mean of 30.0 (s = 6.7) at baseline and 21.5 (s = 10.1)

after three weeks of treatment. The standard deviation was 9.84 for the 21 changes in rating. The authors reported a paired t statistic of 3.96 with df = 20. Show how the authors constructed the t statistic, and report and interpret the P-value for a two-sided test.