Question: The stimulant Ritalin has been shown to increase attention span and improve academic performance in children with ADHD. To demonstrate effectiveness of the drug a researcher selects a sample of n=25 children diagnosed with the disorder and measures each child`s attention span before and after taking the drug. The data show an average increase of attention span of M_D =6.8 minutes with a standard deviation of s=5.5.

a. Is this result sufficient to conclude that Ritalin has a significant effect on attention span? Use a two tailed test with α=.01.

b. Compare r² and estimate Cohen`s d to measure the effect size of the drug.