Evidence-Based Practice for Nurses

The proportion of autism spectrum disorder in this

population of children is 12.66 per 1,000 children.

A rate is a measure of disease frequency in a defined

population over a specified period of time. Rates are used

in epidemiology to describe measures such as incidence

density, crude mortality rates, and fertility rates. When

calculating rate, the numerator is the number of people

affected and the denominator is the entire population. For example, of 6,182

babies born in Lake County in 2016, 52 babies died before reaching their first

birthday. The infant mortality rate in 2016 was

Number ofinfant deathsduring timeperiod

Numberroflivebirthsduringtimeperiod

×=1 000,

52

6 182

1 000 84

,

×=,.per1,000 live births

Prevalence and Incidence

In epidemiology two important rates are reported. These rates are named

prevalence and incidence, and they describe the extent to which a disease is

present in a population.

Prevalence describes the number of existing cases of disease in a population. It is

an indicator of the extent of a health problem in a population and is used for plan-

ning healthcare needs for communities. Prevalence may be expressed as a number,

percentage, proportion, or rate. Prevalence rate per 100,000 at a specified time is

Prevalence=Number ofexistingcasesofadiseasee

Totalpopulation

×100 000,

For example, in 2016, of the 82,692,308 people younger than age 20 years,

215,000 people had diabetes. The prevalence of diabetes in the United States

in 2016 in this age group was

Number ofexistingcases

Totalpopulation

×100 00 ,00=

215 000

82 692 308

, 100 000 0 0026 100 000 260

,,

×=×=,. , perr 100,000

There are two different terms used to describe prevalence. Point prevalence describes

the number of existing cases of disease in a population at a particular point in time.

For example, one might calculate the point prevalence of flu on a college campus

today and then compare that to the point prevalence of flu on the same day in the

previous year. Period prevalence denotes the number of existing cases of disease in

a population during a specified period of time. The time period might be a week,

FYI

Prevalence describes the number of exist-

ing cases of disease in a population. It is an

indicator of the extent of a health problem

in a population and is used for planning

healthcare needs for communities.

KEY TERMS

rate: A measure of

disease frequency

in a defined

population over a

specified period of

time

prevalence: The

number of existing

cases of disease

present in the

population

point prevalence:

The number of

existing cases

of disease in a

population at a

particular point

in time

198 CHAPTER 8 Epidemiologic Designs: Using Data to Understand Populations