Evidence-Based Practice for Nurses

a month, a year, or a number of years. For example, nurses at
a campus health center may calculate the period prevalence
of flu from the previous year’s flu season to make decisions
about how many flu shots to purchase for the current year.

Incidence describes the number of new cases of a disease
in a population during a specified period of time. Incidence
rate is a measure of disease occurrence and is used for investigating the causes of
disease. Nurses use incidence rates to investigate risk of disease in populations or
subgroups related to many factors such as age, gender, occupation, and exposures.
Incidence is calculated by dividing the number of new cases that occur during a
time period by the number of individuals who are at risk. It is important to un-
derstand that the population at risk is different from the entire population. This
is because once people are diagnosed with a disease, they are no longer at risk for
acquiring the disease. Therefore, to determine the population at risk, the number
of known cases must be subtracted from the population. Incidence rates involve a
multiplier (i.e., 1,000 or 100,000) that is relevant to the phenomenon.

Incidence Number ofNEWcases of adisease

Total

=

ppopulationat riskof developingthat disease

×mmultiplier

For example, between 2000 and 2016, the prevalence of persons living with Lyme
disease was 970. In 2016, of the 6,483,802 people living in Indiana, there were 81
confirmed new cases of Lyme disease. Because there were 970 people living with
Lyme disease who were no longer at risk for developing that disease, those individuals
must be subtracted from the 6,483,802 people living in Indiana. The population at
risk is 6,482,832. The incidence of Lyme disease in Indiana in 2016 was

Number ofNEWcases

Totalpopulationatrisk

× 100 ,,

,, ,,

000 81

6 483 802 907

81

6 482 832

=

−

=

= 0.000012 × 100,000 = 1.25 per 100,000

FYI

Incidence rate is a measure of disease occur-

rence and is used for investigating the causes of

disease. Nurses use incidence rates to investigate

risk of disease in populations or subgroups.

KEY TERMS

period prevalence:

The number of

existing cases

of disease in a

population during

a specified period

of time

incidence: The

number of new

cases of a disease in

a population during

a specified period

of time

Match the following:

1. Count data a. measure of disease frequency in a defined population over a
   specified period of time


2. Incidence b. the number of existing cases of disease in a population


3. Prevalence c. the raw number of health phenomena under investigation


4. Rate d. the number of new cases of disease in a population

TEST YOUR KNOWLEDGE 8-3

How did you do? 1. c; 2. d; 3. b; 4. a

8.3 Measures of Disease Frequency 199