Measures of Disease
Frequency
Nigel Paneth
FRACTIONS USED IN DESCRIBING
DISEASE FREQUENCY
RATIO
A fraction in which the numerator is not
part of the denominator.
e.g. Fetal death ratio:
Fetal deaths/live births.
Fetal deaths are not included among live
births, by definition.
PROPORTION
A fraction in which the numerator is part of
the denominator.
e.g. Fetal death rate: Fetal deaths/all births
All births includes both live births and fetal
deaths.
– Synonyms for proportions are: a risk
and, (if expressed per 100) a
percentage.
– Most fractions in epidemiology are
proportions.
RATE
Ideally, a proportion in which
change over time is considered,
but in practice, often used
interchangeably with proportion,
without reference to time, (as I did
previously for fetal death rate).
MEASURES OF DISEASE
FREQUENCY
PREVALENCE RATE
Divided into two types:
1. Point prevalence rate
2. Period prevalence rate
POINT PREVALENCE RATE
Proportion of individuals in a
specified population at risk who
have the disease of interest at a
given point in time.
PERIOD PREVALENCE RATE
Proportion of individuals in a
specified population at risk who
have the disease of interest over a
specified period of time.
For example:
• annual prevalence rate
• lifetime prevalence rate.
(When the type of prevalence rate is not
specified it is usually point prevalence, or its
closest practical approximation)
INCIDENCE RATE
Like prevalence, divided into
two types:
1. Cumulative incidence rate
2. Incidence density
1. Cumulative incidence rate:
Number of new cases of disease
occurring over a specified period
of time in a population at risk at the
beginning of the interval.
EXAMPLE OF CUMULATIVE
INCIDENCE RATE
If we count all new cases of influenza
occurring in MSU undergraduates
from September 1, 1997 - August 31,
1998, and we take as the
denominator all undergraduates
enrolled in September 1, 1997, we
would be describing the cumulative
incidence rate of influenza.
2. Incidence density:
Number of new cases of disease
occurring over a specified period
of time in a population at risk
throughout the interval.
The numerator does not differ between the
two types of incidence
However, the denominator can differ in
incidence density from cumulative
incidence because it takes account of
(in the example):
•Students who left school during the year
•Students who died
•Students who had influenza once and will not
have it again the same season
•Students who entered school later in the year
Incidence density requires us to add
up the period of time each individual
was present in the population, and
was at risk of becoming a new case
of disease.
Incidence density characteristically
uses as the denominator personyears at risk. (Time period can be
person-months, days, or even hours,
depending on the disease process
being studied.)
The mortality rate is a type of
incidence rate which we will
discuss in more detail next
week. It is the most widely used
public health measure, but it has
limitations.
USES OF INCIDENCE AND
PREVALENCE
1. Incidence is generally used for acutely
acquired diseases, prevalence is used for
more permanent states, conditions or
attributes of ill-health.
2. Incidence is more important when thinking
of etiology of the disorder, prevalence when
thinking of societal burden of the disorder
including the costs and resources
consumed as a result of the disorder.
3. Incidence always requires a duration,
prevalence may or may not.
4. In incidence, the unit of analysis is the
event, in prevalence, it is the person.
Thus incidence may exceed 100% (e.g.
annual incidence of colds) unless a
convention is adopted to count only first
episodes of an illness that can occur
more than once.
5. Prevalence can never exceed 100%.
6. Incidence generally requires an initial
disease-free interval before counting
starts, because incidence is measured
only in those at-risk of disease.
USES OF INCIDENCE DENSITY AND
CUMULATIVE INCIDENCE
• Incidence density gives the best
estimate of the true risk of acquiring
disease at any moment in time.
• Cumulative incidence gives the
best estimate of how many people
will eventually get the disease in an
enumerated population.
RELATIONSHIP BETWEEN
INCIDENCE AND PREVALENCE
In a STEADY STATE (i.e. if
incidence is not changing, and the
population is stable)
Prevalence rate = incidence rate
times the duration of disease
(P = I x D)