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The stimulant Ritalin has been shown to increase attention span and improve academic performance in children with ADHD (Evans, Pelham, Smith, et al., 2001). To demonstrate the effectiveness of the drug, a researcher selects a sample of n = 25 children diagnosed with the disorder and measure 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 ∝ = .01.

b. Compute r^2 and estimate Cohen's d to measure the effect size of the drug.

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