Explain why XXXXXXXXXXis a nonlinear mean function. Describe in words what this mean function specifies. Allison and Cicchetti XXXXXXXXXXprovided data on the typical sleeping habits of mammal species....

Explain why (11.21) is a nonlinear mean function. Describe in words what this mean function specifies.

Allison and Cicchetti (1976) provided data on the typical sleeping habits of mammal species. Of interest here is ts, the total hours of sleep per day; Body Wt, the typical body weight of the species; and D, a discrete variable with five values, from D = 1, animals with low danger to d = 5 for animals with very high danger. Fit the mean function

To get starting values, fit the concurrent regression model with γ = 0. The estimate of γ will be very highly variable, as is often the case with centering parameters like γ in this mean function.