# Introduction

We search for the determinants of health outcomes, first, by relying on descriptive epidemiology to generate hypotheses about associations between exposures and outcomes, and, second, by employing analytical epidemiology to more rigorously assess hypotheses by drawing samples of people and comparing groups to determine whether health outcomes differ based on exposure status. If individuals with a given exposure are found to have a greater probability of developing a particular outcome, it suggests an association, and, conversely, if the groups have the same probability of developing the outcome regardless of their exposure status, it suggests that particular exposure is not associated with a greater risk of disease. In either event one must then consider whether the findings were misleading because of sampling error, bias, or confounding (the issue of validity is one that we will address later); in other words, we must consider alternative explanations that might invalidate our conclusions. In this module we will focus on methods for comparing groups and how to interpret the findings.

# Learning Objectives

After successfully completing this section, the student will be able to:

- Construct a tables for summarizing epidemiologic data.
- Explain how to compare the incidence of disease in two or more groups.
- Define, calculate, and interpret:

- risk ratios and rate ratios
- risk difference and rate difference
- attributable proportion (attributable risk percent) for the exposed
- population attributable risk
- odds ratio

- Compute and interpret excess relative risk.
- Discuss the differences between absolute and relative differences in risk.
- Demonstrate the uses of these measures of association and be able to interpret them.
- Explain what is meant by a "reference group" when multiple exposure groups are being compared and be able to compute and interpret measures of association based on a reference group.