Insomnia and education. Many workers suffer from stress and chronic insomnia. Is insomnia related to education status? Researchers at the Universities of Memphis, Alabama at Birming- ham, and Tennessee investigated this question in the Journal of Abnormal Psychology (February 2005). Adults living in Tennessee were selected to participate in the study using a random-digit tele- phone dialing procedure. In addition to insomnia status (normal sleeper or chronic insomnia), the researchers classi?ed each participant into one of four education categories (college graduate, some college, high school graduate, and high school dropout). The dependent variable (y) of interest to the researchers was a quantitative measure of daytime functioning called the Fatigue Severity Scale (FSS), with values ranging from 0 to 5.

(a) Write a main effects model for E(y) asa func- tion of insomnia status and education level. Construct a graph similar to Figure 5.24 that represents the effects hypothesized by the model.

(b) Write an interaction model for E(y) asa func- tion of insomnia status and education level. Construct a graph similar to Figure 5.24 that represents the effects hypothesized by the model.

(c) The researchers discovered that the mean FSS for people with insomnia is greater than the mean FSS for normal sleepers, but that this difference is the same at all education levels. Based on this result, which of the two models best represents the data?