Whereas part 2 of this book covered basic graphical and statistical methods, part 3 discusses intermediate methods. In chapter 8, we move from describing the relationship between two variables to using regression models to model the relationship between a numerical outcome variable and a set of numeric and/or categorical predictor variables. Modeling data is typically a complex, multistep, interactive process. Chapter 8 provides step-by-step coverage of the methods available for fitting linear models, evaluating their appropriateness, and interpreting their meaning.
Chapter 9 considers the analysis of basic experimental and quasi-experimental designs through the analysis of variance and its variants. Here we’re interested in how treatment combinations or conditions affect a numerical outcome variable. The chapter introduces the functions in R that are used to perform an analysis of variance, analysis of covariance, repeated measures analysis of variance, multifactor analysis of variance, and multivariate analysis of variance. It also discusses methods for assessing the appropriateness of these analyses and visualizing the results.