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Measures
of disease occurrence-an introduction
T Ancelle / A Bosman / D Coulombier / A Moren / P Sudre / M Valenciano/
P Penttinen
Epidemiol ogy
Pasi Penttinen
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Objectives
Define and use Risk
Incidence
- Cumulative incidence (CI)
- Attack rate (AR)
- Incidence rate (IR)
Prevalence
Odds
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RISK
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RISK
Non-technical definition
Vague, culture-dependent
Unexpected, unusual, dangerous/negative
events
Epidemiologic definition
Probability that an event will occur
Estimated by:
Observing events among a population during aspecified time
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Cumulative Incidence (CI)(Incidence Proportion)
Number of NEW cases of disease during a period
Population of interest during this period
Example of bilharziasis in Guadeloupe in 1979:
Population 350,000New cases 1,250Cumulative incidence 3.6/1000 during a year
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Cumulative Incidence
CI assumes that entire population at riskfollowed up for specified time period
xx
xxx
x
xx disease onset
Month 1
Month12
CI = 7/12 per year= 0.58 per year
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Attack Rate (AR)
Cumulative incidence during an outbreakUsually expressed for the entire epidemic period,from the first to the last case
Not really a rate but a proportion!
Ex: Outbreak of cholera in country Xin March 1999
Number of cases = 490 Population at risk = 18,600
Attack rate = 2.6%
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RISK
RISK
= The probability that an event will occur
a proportion
= Cumulative incidence
= Attack rate
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Prevalence(point prevalence)
Number of cases of disease at a specific time
Population of interest at that time
Proportion of a population affected by a diseaseat a given time.
Expressed as a percentage
Example of bilharziasis in Guadeloupe in 1979:
Population 350,000Cases 96,200
Prevalence 27.6%
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A
B
CD
E
90 91 92 93 94 95 96 97 98 99 00 Time at risk
x
x
6.0
6.0
10.08.5
5.0
Total years at risk 35.5
-- time followedx disease onset
Person-time
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A
B
CD
E
90 91 92 93 94 95 96 97 98 99 00 Time at risk
x
x
6.0
6.0
10.08.5
5.0
Total years at risk 35.5
-- time followedx disease onset
IR = 2 / 35.5 person- years
= 0.056 / person-year
Incidence rate (IR)(Incidence density)
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Incidence rate
Number of NEW cases of disease during a period
Total person-time of observation
Rate
Denominator:
- is a measure of time- the sum of each individuals time at risk
and free from disease
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Deaths,Cured,
Lost...
DurationPrevalence
Incidence
Merci Jean-Luc Grenier - CEPIQ98
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Odds
Won Lost Total------------------------------------------------------------------------------------------------------------------------------------------------
EPIET facilitatorteam 1995-2004 14 1 15--------------------------------------------------------------------------------------------------------------------------------------------------
Probability that an event will happen
Probability that an event will not happen
14 / 15Odds of winning = -------------
1 / 15
Odds of winning = 14 : 1 = 14
Odds of not winning = 1 : 14 = 0.07
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Odds of a rare event ~ RISK
The number of goals made by teams during EPIET courses
Facilitators Participants Total------------------------------------------------------------------------------------EPIET courses 140 1 141
1995-2004------------------------------------------------------------------------------------
1 / 141Odds of a part. goal= ------------- = 0.007
140 / 141
Risk (CI) of a part. goal = 1/141 = 0.007
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If its not clear
Epidemiology, an introduction. Rothman KJ.
Epidemiology in Medicine. Henneckens CH,
Buring JE. Edited by Mayrent SL.
Modern infectious disease epidemiology.Giesecke J.
Dictionary of epidemiology. Last J.
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