This document provides an overview and preview of Chapter 9 from the textbook Elementary Statistics. Chapter 9 discusses methods for making statistical inferences when there are two samples or populations, extending the techniques introduced in Chapters 7 and 8 which dealt with single samples or populations. The chapter covers topics like comparing two proportions, comparing two means from independent samples, comparing two dependent samples using matched pairs, and comparing two variances or standard deviations. Examples of applications discussed include comparing weight gain in college freshmen, comparing polio rates in children given a vaccine or placebo, and comparing cholesterol levels in subjects given Lipitor or a placebo.