A study of the times to weaning of breast-fed newborns was presented. This data is available on our web site. Categorical variables which could explain the difference in weaning times are the mothers race (white, black, other), smoking status, and an indicator of whether the mother was in poverty. Continuous variables which could explain outcome are the mother's age at the child's birth, mothers years of education, and the child's year of birth. Using a Cox model with appropriate terms for the mother's race, smoking status and poverty indicator, determine if each of the three continuous covariates would enter the model as a linear function.