Chapter 14

Analyzing Incidence and Prevalence Rates in

Epidemiologic Data

IN THIS CHAPTER

Determining and expressing the prevalence of a condition

Calculating incidence rates and rate ratios, along with their standard errors

Comparing incidence rates between two populations

Estimating sample size needed to compare incidence rates

Epidemiology is the study of the causes of health and disease in human populations. It is sometimes

defined as characterizing the three Ds — the distribution and determinants of human disease

(although epidemiology technically also concerns more positive outcomes, such as human health and

wellness). This chapter describes two concepts central to epidemiology: prevalence and incidence.

Prevalence and incidence are also frequently encountered in other areas of human research as well.

We describe how to calculate incidence rates and prevalence proportions. Then we concentrate on the

analysis of incidence. (For an introduction to prevalence and to learn how to calculate prevalence

ratios, see Chapter 13.) Later in this chapter, we describe how to calculate confidence intervals

around incidence rates and rate ratios, and how to compare incidence rates between two populations.

Understanding Incidence and Prevalence

Incidence and prevalence are two related but distinct concepts. In the following sections, we define

each of these concepts and provide examples. After that, we describe the relationship between

incidence and prevalence.

Prevalence: The fraction of a population with a particular condition

The prevalence of a condition in a population is the proportion of the population that has that

condition at any given moment. It’s calculated by creating a fraction with a numerator and a

denominator. The denominator is the total population eligible to have the condition. The

numerator is the number of individuals from the population who have the condition at a given

time. If you divide this numerator by this denominator, you will calculate the prevalence of the

condition in that population.

Prevalence can be expressed as a decimal fraction, a percentage, or a rate per so many (usually per

1,000, per 10,000, or per 100,000). For example, a 2021 survey found that 11.6 percent of the U.S.

adult population has Type II diabetes. But a rarer outcome — such as a monthly hospitalization rate for

those suffering from influenza — may be expressed as 31.7 per 100,000. The prevalence is expressed