CHAPTER 14 Analyzing Incidence and Prevalence Rates in Epidemiologic Data 193

mean that a member of the population goes from not having the condition to hav-

ing the condition.) Take note that while incidence expresses the number of new

cases of the condition in the numerator, in contrast, prevalence includes all

cases — both new and existing — in the numerator. The denominator for inci-

dence is defined as the number of individuals in the population who could have

had the event multiplied by the interval of time being used. This is also called time

exposed or exposure (E). So, the equation for incidence is the number of observed

events divided by the exposure, which is (E): R = N/E.

Exposure is measured in units of person-time, such as person-days or person-

years. Incidence rates are expressed as the number of cases per unit of person-

time. The unit of person-time is used so that the incidence rate can at least be the

size of a whole number so it is easier to interpret and compare.

The incidence rate should be estimated by counting events over a narrow enough

interval of time so that the number of observed events is a small fraction of the

total population studied. One year is narrow enough for calculating incidence of

Type II diabetes in adults because 0.02 percent of the adult population develops

diabetes in a year. However, one year isn’t narrow enough to be useful when con-

sidering the incidence of an acute condition like influenza. In influenza and other

infectious diseases, the intervals of interest would be in terms of daily, weekly,

and monthly trends. It’s not very helpful to know that 30 percent of the popula-

tion came down with influenza in a one-year period.

Consider City XYZ, which has a population of 300,000 adults. None of them has

been diagnosed with Type II diabetes. Suppose that in 2023, 30 adults from City

XYZ were newly diagnosed with Type II diabetes. The incidence of adult Type II

diabetes in City XYZ would be calculated with a numerator of 30 cases and a

denominator of 300,000 adults in one year. Using the incidence formula, this

works out to 0.0001 new cases per person-year. As described before, in epidemi-

ology, rates are reconfigured to have at least whole numbers so that they are eas-

ier to interpret and envision. For this example, you could express City XYZ’s 2023

adult Type II diabetes incidence rate as 1 new case per 10,000 person-years, or as

10 new cases per 100,000 person-years.

Now imagine another city — City ABC — has a population of 80,0000 adults, and

like with City XYZ, none of them had ever been diagnosed with Type II diabetes.

Now, assume that in 2023, 24 adults from City ABC were newly diagnosed with

Type II diabetes. City ABC’s 2023 incidence rate would be calculated as 24 cases in

80,000 individuals in one year, which works out to 24 80 000

/

,

or 0.0003 new cases

per person-year. To make the estimate comparable to City XYZ’s estimate, let’s

express City ABC’s estimate as 30 new cases per 100,000 person-years. So, the

2023 adult Type II diabetes incidence rate in City ABC — which is 30 new cases per

100,000 person-years — is three times as large as the 2023 adult Type II diabetes