Understanding and Applying Basic Statistical Methods Using R

Chapter 9COMPARING TWO INDEPENDENT GROUPS

Chapters 6 and 7 described how to make inferences about the population mean, and other measures of location, associated with a single population of individuals or things. This chapter extends these methods to situations where the goal is to compare two independent groups. For example, Table 2.1 reports data from a study on changes in cholesterol levels when participants take an experimental drug. Of fundamental interest is how the changes compare to individuals who receive a placebo instead. Section 6.2 described an experiment on the effect of ozone on weight gain among rats. The two groups in this study consisted of rats living in an ozone environment and ones that lived in an ozone-free environment. Do weight gains differ for these groups, and if they do, how might this difference be described? Two training programs are available for learning how to invest in stocks. To what extent, if any, do these training programs differ? How does the reading ability of children who watch 30 hours or more of television per week compare to children who watch 10 hours or less? How does the birth weight of newborns among mothers who smoke compare to the birth weight among mothers who do not smoke? In general terms, if we have two independent variables, how might we compare them?

A basic goal in this chapter is to describe and illustrate methods that can be used to compare groups, and to discuss their relative merits regarding their ability to control ...