The stimulant Ritalin has been shown to increase attention span and improve academic performance in children with ADHD (Evans et al., 2001). To demonstrate the effectiveness of the drug, a researcher selects a sample of n 5 20 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 MD 5 4.8 minutes with a variance of s 2 5 125 for the sample of difference scores.

a. Is this result sufficient to conclude that Ritalin significantly improves attention span? Use a onetailed test with a 5 .05.

b. Compute the 80% confidence interval for the mean change in attention span for the population.