The methods of Chapter 3 are really quite simple. They require little in the way of calculations and are most obviously relevant to the analysis of a single engineering variable. This chapter provides methods that address the more complicated problem of describing relationships between variables and are computationally more demanding. The chapter begins with least squares fitting of a line to bivariate quantitative data and the assessment of the goodness of that fit. Then the line-fitting ideas are generalized to the fitting of curves to bivariate data and surfaces to multivariate quantitative data. The next topic is the summarization of data from full factorial studies in terms of so-called factorial effects. Next, the notion of data transformations is discussed. Finally, the chapter closes with a short transitional section that argues that further progress in statistics requires some familiarity with the subject of probability.