measures
of disease occurrence
Yan He
Capital Medical University

Objectives
Define and use
Ratio
Odds
Rate
Proportion
Define and use
Prevalence
Incidence

Fundamental question
Is a subject
(individual, group, herd, region, country)
diseased
(abnormal, affected)
or not diseased
(normal, unaffected)
?
Most health statistics and epidemiological research
are based on counting of cases

Measurement of disease frequency
Define ‘Disease’
Define population
Define timeframe
DISEASE
FREQUENCY

Case definition
The set of criteria that differentiate
‘
normality
’
from
‘
abnormality
’
Most important step in epidemiology
Clinical/laboratory/epidemiological
May need revision as knowledge advances

Defining
the population
Usually easier than case definition
Equally important
Unit of interest (person, group, nation)
Defined geographically or by other specific criteria
Target, study and sample populations

•
Dichotomous variables
Case –control
Exposed –unexposed
Dead –alive
Ratio, proportion, rate

What, who is in the denominator (
分母
)????
Ratio
Proportion
Rate

Ratio
（比率）
The quotient
（商）
of 2 numbers
Numerator
（分子）
NOT
necessarily INCLUDED
in
the denominator
（分母）
Allows to compare quantities
（量）
of different
nature

= 5 /
2
=
2.5 /
1
#
beds per doctor
850 beds/10 doctors
R = 85 beds for 1 doctor
# participants per facilitator
# inhabitants per latrine

Proportion
（比率）
The quotient of 2 numbers
Numerator
（分子）
NECESSARELY
INCLUDED
（分母）
in the denominator
Quantities have to be of same nature
Proportion always ranges between 0 and 1
Percentage = proportion x 100

2
--- = 0.5 = 50%
4
Persons in denominator reflect population from which
cases (numerator) arose
Proportion of German EPIET in 8 first cohorts
101 trainees, 9 German
Proportion of German trainees = 0.09
Percentage of German trainees = 9%

Rate
The quotient of 2 numbers
Speed of occurrence of an event over time
Observed in 1998
Numerator
number
EVENTS
observed for a given time

2
----- = 0.02 /
year
100
Observed in 1998
Numerator
number
EVENTS
observed for a given time
Denominator
population in which
the events may occur
(
population at risk
)
- includes time

Rate, Example
Mortality rate of tetanus in France in 1995
Tetanus deaths: 17
Population in 1995: 58 million
Mortality rate =
Rate may be expressed in any power of 10
100, 1,000, 10,00,
100,000
0.029/100,000/year

Measures of disease occurence
Epidemiology :
«
Study of the occurrence of illness
»
Two types of measures :
Prevalence
Measures population disease status
Incidence
Assess frequency of disease onset
Cumulative incidence
or incidence proportion
Incidence density
or incidence rate

incidence
Incidence quantifies the number of new evens or cases of
disease that
develop in a population of individuals
at risk
during a special time interval.

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