Computation and Measurement of Risk - Mathematics and Statistics for the Environment - page 192

Two examples of prevalence fractions are
Number of womenintownofFraminghamwho
reportedhavingheartdiseaseinrecenthealthsurvey
Estimated numberofwomen residentsofFramingham duringsameperiod
Estimated numberofwomen smokers in StateAaccording to 2004 BehaviorRiskFactor Survey
Estimated numberofwomen living in State AonJuly1, 2004
■ EXAMPLE 8.11
Problem: In a survey of 1150 women who gave birth in Maine in 2000, a total of 468 reported tak-
ing a multivitamin at least 4 times a week during the month before becoming pregnant (Williams et
al., 2003). Calculate the prevalence of frequent multivitamin use in this group.
Solution:
Numerator = 468 multivitamin users
Denominator = 1150 women
Prevalence = (448/1150) × 100 = 0.407 × 100 = 40.7%
8.3.3.1 Properties and Uses of Prevalence
Prevalence and incidence are frequently confused. Prevalence refers to proportion of persons who
have a condition at or during a particular time period, whereas incidence refers to the proportion or
rate of persons who develop a condition during a particular time period. So, prevalence and inci-
dence are similar, but prevalence includes new and preexisting cases, whereas incidence includes
new cases only. The key difference is in their numerators:
• Numerator of incidence = New cases that occurred during a given time period.
• Numerator of prevalence = All cases present during a given time period.
The numerator of an incidence proportion or rate consists only of persons whose illness began dur-
ing the specified interval. The numerator for prevalence includes all persons ill from a specified
cause during the specified interval regardless of when the illness began . It includes not only new
cases but also preexisting cases representing persons who remained ill during some portion of the
specified interval.
Prevalence is based on both incidence and duration of illness. High prevalence of a disease within
a population might reflect high incidence or prolonged survival without cure or both. Conversely,
low prevalence might indicate low incidence, a rapidly fatal process, or rapid recovery. Prevalence
rather than incidence is often measured for chronic diseases such as diabetes or osteoarthritis that
have long duration and dates of onset that are difficult to pinpoint.
Figure 8.1 shows 10 new cases of illness over about 15 months in a population of 20 persons.
Each horizontal line represents one person. The down arrow indicates the date of onset of illness.
The solid line represents the duration of illness. The up arrow and the cross represent the date of
recovery and date of death, respectively. Use Figure 8.1 to solve the problems presented in Example
8.12, Example 8.13, and Example 8.14.
■ EXAMPLE 8.12
Problem: Calculate the incidence rate from October 1, 2004, to September 30, 2005, using the
midpoint population (population alive on April 1, 2005) as the denominator. Express the rate per
100 population.