192 PART 4 Comparing Groups

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 eli-

gible 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 as

the result of a calculation from this fraction, but stated as a rate so that it is easy

to envision. It would be hard to envision that 0.0317 percent of influenza sufferers

were hospitalized in one month. On the other hand, it is much easier to envision

almost 32 people from a town with a population of 100,000 being hospitalized in

one month — provided you also envision that everyone in the town had influenza.

Because prevalence is a proportion, it’s analyzed in exactly the same way as any

other proportion. The standard error (SE) of a prevalence ratio can be estimated

by the formula in Chapter 13. Confidence intervals (CIs) for a prevalence estimate

can be obtained from exact methods based on the binomial distribution or from

formulas based on the normal approximation to the binomial distribution. Also,

prevalence can be compared between two or more populations using the chi-

square or Fisher Exact test. For this reason, the remainder of this chapter focuses

on how to analyze incidence rates.

Incidence: Counting new cases

The incidence of a condition is the rate at which new cases of that condition appear

in a population. Incidence is generally expressed as an incidence rate (R), which —

like prevalence  — is a fraction. The numerator for incidence is defined as the

number of observed events (N) in a particular time period. (Consider an event to