CHAPTER 11 Comparing Average Values between Groups 141

Chapter 11

Comparing Average

Values between Groups

C

omparing average values between groups of numbers is part of almost all

biostatistical analyses, and over the years, statisticians have developed

dozens of tests for this purpose. These tests include several different fla-

vors of the Student t test, analyses of variance (ANOVA), and a dizzying collection

of tests named after the men who popularized them, including Welch, Wilcoxon,

Mann-Whitney, and Kruskal-Wallis, to name just a few. The multitude of tests is

enough to make your head spin, which leaves many researchers with the uneasy

feeling that they may be using the wrong statistical test on their data.

In this chapter, we guide you through the menagerie of statistical tests for com-

paring groups of numbers. We start by explaining why there are so many tests

available, then guide you as to which ones are right for which situations. Next, we

show you how to execute these tests using R software, and how to interpret the

output. We focus on tests that are usually provided by modern statistical programs

(like those discussed in Chapter  4, which also explains how to install and get

started with R).

IN THIS CHAPTER»

» ^{Determining which tests should be}

used in different situations»

» ^{Preparing your data, running tests,}

and interpreting the output»

» ^{Estimating the sample size you need}

to compare average values