Transcript Epidemiologic Measures

Epidemiologic Measures
Afshin Ostovar
Bushehr University of Medical Sciences
Bushehr, 2012
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Frequency Distributions
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line listing
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frequency distribution
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Frequency Measures
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Ratios
Proportions
Rates
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Formula
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Ratio
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In a ratio, the values of x and y
may be completely independent, or
x may be included in y.
Exp:
Male/Female
Male/All
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Proportion
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A proportion, the second type of
frequency measure used with
dichotomous variables, is a ratio in
which x is included in y.
Exp:
Male/Female
Male/All
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Rate
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Rate, is often a proportion, with an added
dimension: it measures the occurrence of
an event in a population over time. The
basic formula for a rate is as follows:
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Three important aspects of this
formula:
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The persons in the denominator must reflect
the population from which the cases in the
numerator arose.
The counts in the numerator and denominator
should cover the same time period.
In theory, the persons in the denominator
must be “at risk” for the event, that is, it
should have been possible for them to
experience the event.
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a. Point prevalence on October 1, 1990?
b. Period prevalence, October 1, 1990 to September 30, 1991?
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Attack Rate
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An attack rate is a variant of an incidence rate,
applied to a narrowly defined population
observed for a limited time, such as during an
epidemic.
The attack rate is usually expressed as a
percent, so 10n equals 100.
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Attack Rate
Example
Of 75 persons who attended a church picnic, 46
subsequently developed gastroenteritis.
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Secondary Attack Rate

frequency of new cases of a disease
among the contacts of known cases.
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Person-time Rate
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Measures of Association
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Risk Ratio (Relative Risk)
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Rate Ratio (Relative Rate)
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Odds Ratio (cross-product ratio)
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Risk Ratio (Relative Risk)
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A risk ratio, or relative risk, compares the risk of
some health-related event such as disease or
death in two groups.
The two groups are typically differentiated by
demographic factors such as sex (e.g., males
versus females) or by exposure to a suspected risk
factor (e.g., consumption of potato salad or not).
Often, you will see the group of primary interest
labeled the “exposed” group, and the comparison
group labeled the “unexposed” group.
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Risk Ratio (Relative Risk)
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Risk Ratio (Relative Risk)
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Rate Ratio
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A rate ratio compares two groups in terms of
incidence rates, person-time rates, or mortality
rates.
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Like the risk ratio, the two groups are typically
differentiated by demographic factors or by
exposure to a suspected causative agent.
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Rate Ratio
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Rate Ratio
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Odds Ratio (cross-product ratio)
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a = number of persons with disease and with exposure of interest
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b = number of persons without disease, but with exposure of interest
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c = number of persons with disease, but without exposure of interest
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d = number of persons without disease and without exposure of
interest
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a + c = total number of persons with disease (“cases”)
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b+d=
total number of persons without disease (“controls”)
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Odds Ratio (cross-product ratio)
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Attributable Proportion
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The attributable proportion, also known as the
attributable risk percent, is a measure of the public
health impact of a causative factor.
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Thus, the attributable proportion is the proportion of
disease in an exposed group attributable to the
exposure. It represents the expected reduction in
disease if the exposure could be removed (or never
existed).
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Attributable Proportion
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Attributable Proportion
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Mortality Rates
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Natality Frequency Measures
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Direct Standardization
0.365
0.362
0.36
HTN Rate
0.355
0.35
0.345
0.34
0.335
0.33
0.3341
0.325
0.32
Phase 1
Phase 2
HTN Rates in Two Phases of MONICA Study, Bushehr
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Direct Standardization, Cont.
HTN Rates in Two Phases of MONICA Study, Bushehr
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Age Group
(Years)
Phase 1
Phase 2
25-34
15.2%
10.4%
35-44
29.4%
20.3%
45-54
50.7%
43.0%
55-66
64.8%
56.8%
>67
85.7%
70.8%
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Direct Standardization, Cont.
Hypertension rate change in different age groups in the first and second phases
90.0%
80.0%
70.0%
60.0%
25-34
50.0%
35-44
40.0%
45-54
30.0%
55-66
>67
20.0%
10.0%
.0%
Phase 1
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Phase 2
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Direct Standardization
0.365
0.362
0.36
HTN Rate
0.355
0.35
0.345
0.34
0.335
0.33
0.3341
0.325
0.32
Phase 1
Phase 2
HTN Rates in Two Phases of MONICA Study, Bushehr
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Direct Standardization
Age Distribution in Two Phases of MONICA Study, Bushehr
1600
1400
1200
1000
Phase 1
800
Phase 2
600
400
200
0
25-34
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35-44
45-54
55-66
>67
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Direct Standardization
0.38
0.36
0.365458
0.34
0.32
0.3
0.303358
0.28
0.26
0.24
0.22
0.2
Phase 1
Phase 2
Directly Age Adjusted HTN Rates in Two Phases of MONICA Study,
Bushehr
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Indirect Standardization
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Observed / Expected
Occupational Studies
Standard Mortality Ratio
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