Table of Contents ix

Using Statistical Tests for Comparing Averages. . . . . . . . . . . . . . . . . . 146

Surveying Student t tests. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147

Assessing the ANOVA. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152

Running nonparametric tests. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157

Estimating the Sample Size You Need for Comparing Averages. . . . 158

Using formulas for manual calculation . . . . . . . . . . . . . . . . . . . . . . 158

Software and web pages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158

CHAPTER 12: Comparing Proportions and Analyzing

Cross-Tabulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159

Examining Two Variables with the Pearson Chi-Square Test. . . . . . . 161

Understanding how the chi-square test works. . . . . . . . . . . . . . . . 161

Pointing out the pros and cons of the chi-square test. . . . . . . . . . 167

Modifying the chi-square test: The Yates

continuity correction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168

Focusing on the Fisher Exact Test. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169

Understanding how the Fisher Exact test works . . . . . . . . . . . . . . 169

Noting the pros and cons of the Fisher Exact test. . . . . . . . . . . . . 170

Calculating Power and Sample Size for Chi-Square and

Fisher Exact Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171

CHAPTER 13: Taking a Closer Look at Fourfold Tables. . . . . . . . . . . . 173

Focusing on the Fundamentals of Fourfold Tables . . . . . . . . . . . . . . . 174

Choosing the Correct Sampling Strategy. . . . . . . . . . . . . . . . . . . . . . . . 175

Producing Fourfold Tables in a Variety of Situations. . . . . . . . . . . . . . 176

Describing the association between two binary variables . . . . . . 177

Quantifying associations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178

Evaluating diagnostic procedures. . . . . . . . . . . . . . . . . . . . . . . . . . . 183

Investigating treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187

Looking at inter- and intra-rater reliability. . . . . . . . . . . . . . . . . . . .188

CHAPTER 14: Analyzing Incidence and Prevalence Rates in

Epidemiologic Data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191

Understanding Incidence and Prevalence. . . . . . . . . . . . . . . . . . . . . . . 192

Prevalence: The fraction of a population with a

particular condition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192

Incidence: Counting new cases. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192

Understanding how incidence and prevalence are related . . . . . 194

Analyzing Incidence Rates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194

Expressing the precision of an incidence rate. . . . . . . . . . . . . . . . . 194

Comparing incidences with the rate ratio. . . . . . . . . . . . . . . . . . . . 195

Calculating confidence intervals for a rate ratio. . . . . . . . . . . . . . . 196

Comparing two event rates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196

Comparing two event counts with identical exposure. . . . . . . . . . 197

Estimating the Required Sample Size. . . . . . . . . . . . . . . . . . . . . . . . . . . 198