Are severe psychiatric disorders related to biological factors that can be physically measured? Refer to Luxenberg et al., Am. J. Psychiatry, v. 145, no. 9. In this study X-ray computer tomography (CT) was used to measure brain volumes of patients with obsessive-compulsive disorder (OCD) and healthy volunteers.

• Construct a 99% confidence interval estimate of the difference in mean brain volumes for the OCD and the healthy cohorts.
• What does this confidence interval suggest about the difference between these two population means?
• Does it seem that OCD has a biological basis?

Data: control (healthy) group: N = 10, mean brain volume = 0.45, standard deviation = 0.08; OCD group: N = 10, mean brain volume = 0.34, standard deviation = 0.08

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