An experiment was conducted at the University of California-Berkeley to study the effect of psychological environment on the anatomy of the brain. A group of 19 rats was randomly divided into two groups. Twelve animals in the treatment group lived together in a large cage furnished with playthings that were changed daily; animals in the control group lived in isolation with no toys. After a month, the experimental animals were killed and dissected. Table E2.19 gives the cortex weights (the thinking part of the brain) in milligrams. Calculate separately for each group the mean x, variance s2, and standard deviation s of the cortex weight.

TABLE E2.19