Epidemiology Lab 2: Frequency measures used in Epidemiology
Objectives:
After studying this lesson and answering the questions in the exercises, you will be able to
calculate and interpret the following epidemiologic measures:
1.
Ratio
2.
Proportion
3.
Incidence proportion (attack rate)
4.
Incidence rate
5.
Prevalence
Background
:
Frequency Measures
A measure of central location provides a single value that summarizes an entire distribution of
data. In contrast, a frequency measure characterizes only part of the distribution. Frequency
measures compare one part of the distribution to another part of the distribution, or to the entire
distribution. Common frequency measures are
ratios
,
proportions
, and
rate
s
. All three
frequency measures have the same basic form:
(Numerator /
denominator) x 10
n
Recall that:
10
0
= 1 (anything raised to the 0 power equals 1)
10
1
= 10 (anything raised to the 1st power is the value itself)
10
2
= 10 x 10 = 100
10
3
= 10 x 10 x 10 = 1,000
So the fraction of (numerator/denominator) can be multiplied by 1, 10, 100, 1000, and so on.
This multiplier varies by measure and will be addressed in each section.
Ratio
Definition of ratio
A ratio is the relative magnitude of two quantities or a comparison of any two values. It is
calculated by dividing one interval- or ratio-scale variable by the other. The numerator and
denominator need not be related. Therefore, one could compare apples with oranges or apples
with number of physician visits.

Method for calculating a ratio
Number
∨
rateof events,items , persons ,etc
∈
agroup
Number
∨
rateof events,items, persons ,etc
∈
another group
After the numerator is divided by the denominator, the result is often expressed as the result “to
one” or written as the result “:1.” Note that in certain ratios, the numerator and denominator are
different categories of the same variable, such as males and females, or persons 20–29 years and
30–39 years of age. In other ratios, the numerator and denominator are completely different
variables, such as the number of hospitals in a city and the size of the population living in that
city.
Proportion
Definition of proportion
A proportion is the comparison of a part to the whole. It is a type of ratio in which the numerator
is included in the denominator. You might use a proportion to describe what fraction of clinic
patients tested positive for HIV, or what percentage of the population is younger than 25 years of
age. A proportion may be expressed as a decimal, a fraction, or a percentage.
Method for calculating a proportion
Totalnumber of persons
∨
events,of whichthe numeratoris a
⊂
¿
x
10
n
Number of persons oneventswith a particular characteristic
¿
For a proportion, 10
n
is usually 100 (or n=2) and is often expressed as a percentage.

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- Summer '14
- Kissoon
- Epidemiology