Evidence-Based Practice for Nurses

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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
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