Evidence-Based Practice for Nurses

8.3 Measures of Disease Frequency

At the end of this section, you will be able to:

‹ Describe count data, ratios, proportions, and rates

‹ Define and compute prevalence

‹ Define and compute incidence

Measures of disease frequency are used to quantify health outcomes to describe
and compare populations. Several types of epidemiologic measures are used to
calculate disease frequency. These include count data, ratios, proportions, and rates.

Epidemiologic Measures

The simplest measure used is count data. Count data refer to the raw number
of health phenomena under investigation and would include health events
such as births, cases of a disease, and deaths. Count data are not particularly
useful when comparing populations of different sizes. For example, a country
with a large population will have more births than will a country with a small
population regardless of other factors associated with birth rate.

When populations differ in size, epidemiologists use ratios to compare and
contrast health outcomes across populations. A ratio describes a mathematical
relationship between two numbers. The formula is a/b, and the numerator and
the denominator do not necessarily have to have a specified association. For
example, of 1,000 patients with acute myocardial infarction (AMI), 600 were
male and 400 were female. The sex ratio for AMIs is

Number ofmalecases

Number offemalecases

=^600

4000

= 15 .:1maletofemale

Proportions and rates are all types of ratios. A proportion is a type of ratio in
which the numerator is included in the denominator. The formula for calculating
a proportion is a/a + b multiplied by 1,000. In a proportion, the numerator and
denominator have an association. The numerator is the number of cases, deaths,
or events, and the denominator is the population being studied. Proportions
are used in epidemiology to describe measures such as prevalence, cumulative
incidence, case fatality rates, and attack rates. For example, of 300,000 children,
3,800 were diagnosed with an autism spectrum disorder and 296,200 were
not. The proportion for children diagnosed with autism spectrum disorders is

a

ab+

×=

+

1 000 3 800 ×=

3 800

1 000 3 800

300 00

, ,

,

, ,

296,200 , 00

× 1 , 000

KEY TERMS

count data: The raw

number of health

phenomena under

investigation in

epidemiology

ratio: The

highest level of

measurement that

involves numeric

values that begin

with an absolute

zero and have

equal intervals;

in epidemiology

a mathematical

relationship

between two

numbers

proportion: A type

of ratio where

the numerator is

included in the

denominator

=×0.01266 1 000 12 66,.= per0 10 , 0children

8.3 Measures of Disease Frequency 197