# Lesson 3: Measures of Risk

This is an online version of a printed textbook. It is not intended to be an online course.

Refer to the book or to the electronic PDF version (511 pages) for printable versions of text, figures, and tables.

## Overview

Lesson 2 described measures of central location and spread, which are useful for summarizing continuous variables. However, many variables used by field epidemiologists are categorical variables, some of which have only two categories — exposed yes/no, test positive/negative, case/control, and so on. These variables have to be summarized with frequency measures such as ratios, proportions, and rates. Incidence, prevalence, and mortality rates are three frequency measures that are used to characterize the occurrence of health events in a population.

## Objectives

After studying this lesson and answering the questions in the exercises, you will be able to:

- Calculate and interpret the following epidemiologic measures:
- Ratio
- Proportion
- Incidence proportion (attack rate)
- Incidence rate
- Prevalence
- Mortality rate

- Choose and apply the appropriate measures of association and measures of public health impact

## Major Sections

- Section 1: Frequency Measures
- Section 2: Morbidity Frequency Measures
- Section 3: Mortality Frequency Measures
- Section 4: Natality (Birth) Measures
- Section 5: Measures of Association
- Section 6: Measures of Public Health Impact
- Summary and References
- Exercise Answers
- Self-Assessment Quiz
- Answers to Self-Assessment Quiz