Geoscience Reference
In-Depth Information
For non-diabetic women:
Numerator
=
=
511
Denominator
36,653
Person-time
rate
=
=
511/36,653
0.0139 deaths perperson-y
ear
13.9 deaths per 1000 person-years
=
■
EXAMPLE 8.10
Problem:
In 2003, 44,232 new cases of acquired immunodeficiency syndrome (AIDS) were reported
in the United States (CDC, 2005). The estimated mid-year population of the United States in 2003
was approximately 290,809,777 (U.S. Census Bureau, 2006). Calculate the incidence rate of AIDS
in 2003.
Solution:
Numerator
=
44,232 new cases of AIDS
Denominat
or
=
290,809,777 estimated mid-year populati
on
n
=
=
10
100 000
,
Incidencerate 44,232/290,80
9,777) 100,000
= 15.21 new cases of AIDS p
×
er 100,000 population
8.3.3 p
revalenCe
Prevalence
, sometimes referred to as
prevalence rate
, is the proportion of persons in a population
who have a particular disease or attribute at a specified point in time or over a specified period of
time. Prevalence differs from incidence in that prevalence includes all cases, both new and preexist-
ing, in the population at the specified time, whereas incidence is limited to new cases only.
Point
prevalence
refers to the prevalence measured at a particular point in time. It is the proportion of
persons with a particular disease or attribute on a particular date.
Period prevalence
refers to preva-
lence measured over an interval of time. It is the proportion of persons with a particular disease or
attribute at any time during the interval.
• Method for calculating prevalence of disease:
Allnew andpre-existing cases duringagiven time period
Population duringthe same timeperiod
×10
n
• Method for calculating prevalence of an attribute:
Persons having aparticular attribute duringgagiven timeperiod
Population duringthe same time period
×10
n
The value of 10
n
is usually 1 or 100 for common attributes. The value of 10
n
might be 1000, 100,000,
or even 1,000,000 for rare attributes and for most diseases.
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