4.7 Variable variance

We have been using the linear motif to model the mean of a distribution and, in the previous section, we used it to model interactions. In statistics, it is said that a linear regression model presents heteroskedasticity when the variance of the errors is not constant in all the observations made. For those cases, we may want to consider the variance (or standard deviation) as a (linear) function of the dependent variable.

The World Health Organization and other health institutions around the world collect data for newborns and toddlers and design growth chart standards. These charts are an essential component of the pediatric toolkit and also a measure of the general well-being of populations to formulate health-related policies, plan interventions, and monitor their effectiveness. An example of such data is the lengths (heights) of newborn/toddler girls as a function of their age (in months):

Code 4.9

data = pd.read_csv("data/babies.csv") ...