Geoscience Reference
In-Depth Information
TABLE 8.3
Frequently Used Measures of Morbidity
Measure
Numerator
Denominator
Incidence proportion
(or attack rate or risk)
Number of new cases of disease during specified
time interval
Population at start of time interval
Secondary attack rate
Number of new cases among contacts
Total number of contacts
Incidence rate (or
person-time rate)
Number of new cases of disease during specified
time interval
Summed person-years of observation or
average population during time interval
Point prevalence
Number of current cases (new and preexisting)
at a specified point in time
Population at the same specified point
Period prevalence
Number of current cases (new and preexisting)
over a specified period of time
Average or mid-interval population
8.3 MORBIDITY FREQUENCY MEASURES
Morbidity has been defined as any departure, subjective or objective, from a state of physiological
or psychological well-being. In practice, morbidity encompasses disease, injury, and disability. In
addition (although for this topic the term refers to the number of persons who are ill), it can also
be used to describe the periods of illness that these persons experience, or the duration of these
illnesses (Last, 2001). Measures of morbidity frequency characterize the number of persons in a
population who become ill (incidence) or are ill at the given time (prevalence). Commonly used
measures are listed in Table 8.3. Incidence refers to the occurrence of new cases of disease or injury
in a population over a specified period of time. Although some environmental health practitioners
use incidence to mean the number of new cases in a community, others use incidence to mean the
number of new cases per unit of population. Two types of incidence are commonly used— incidence
proportion and incidence rate .
8.3.1 i nCidenCe p roportion or r isK
Incidence proportion is the proportion of an initially disease-free population that develops disease,
becomes injured, or dies during a specified (usually limited) period of time. Synonyms include
attack rate, risk, probability of getting diseases, and cumulative incidence. Incidence proportion is a
proportion because the persons in the numerator, those who develop disease, are all included in the
denominator (the entire population). Incidence proportion (risk) is calculated by
Number of new cases of diseaseorinjury during specified period
Size of populationat startofperiod
EXAMPLE 8.5
Problem : In the study of diabetics, 100 of the 189 diabetic men died during the 13-year follow-up
period. Calculate the risk of death for these men.
Solution:
Numerator = 100 deaths among the diabetic men
Denominator = 189 diabetic men
10 n = 10 2 = 100
Risk = (100/189) × 100 = 52.9%
 
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