Using R for Introductory Statistics : John Verzani

Using R for introductory statistics 31611.3 ANCOVAAn analysis of covariance (ANCOVA) is the term given to models where bothcategorical and numeric variables are used as predictors. Performing an ANCOVA in R isalso done using lm().■ Example 11.8: Birth weight by mother’s weight and smoking historyIn Example 11.6 we performed an analysis of variance of a baby’s birth weightmodeled by whether the mother smoked. In this example, we also regress on the numericmeasurement of the mother’s weight. First we make a plot, marking the points withdifferent characters depending on the value of smoke. As smoke is stored as a numericvariable, the different plot symbols for those numbers are used.> plot(wt ~ wt1, data=babies, pch=smoke, subset=wt1 res=lm(wt ~ wt1+factor(smoke), data=babies,+ subset=wt1 < 800)> summary(res)Call:lm(formula=wt ~ wt1+factor(smoke), data=babies,subset=wt1