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Measures of disease occurrence T Ancelle / A Bosman / D Coulombier / A Moren / P Sudre / M Valenciano/ P Penttinen Peter Kreidl

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Objectives Define and use –Risk - Cumulative incidence (CI) - Attack rate (AR) - Rate - Incidence rate (IR) –Odds –Prevalence

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RISK

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Risk

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RISK Non-technical definition –Vague, culture-dependent –Unexpected, unusual, dangerous/negative events

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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 a specified time

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Cumulative Incidence (CI) (Incidence Proportion) Number of NEW cases of disease during a period Population at the beginning of the period

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Cumulative Incidence (CI) (Incidence Proportion) Number of NEW cases of disease during a period Population at the beginning of the period Ex: Bilharziosis in Guadeloupe in 1979: Population 350,000 New cases 1,250 Cumulative incidence= 0.0036 per year = 0.36 % per year = 3.6 new cases / 1000 during a year

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x x x x x x x x disease onset Month 1 Month12 Cumulative Incidence Population = 12 Diseased = 7

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x x x x x x x x disease onset Month 1 Month12 Cumulative Incidence CI = 7/12 = 0.58 per year = 58% per year

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x x x x x x x x disease onset Month 1 Month12 Cumulative Incidence CI = 7/12 = 0.58 per year = 58% per year CI assumes that the entire population at risk is followed up for the same time period

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Attack Rate (AR) Cumulative incidence during an outbreak Expressed for the entire epidemic period, from the first to the last case Not really a rate but a proportion!

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Attack Rate (AR) Cumulative incidence during an outbreak 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 X in March 1999 –Number of cases 490 –Population 18,600 –Attack rate 2.6%

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RISK The probability that an event will occur Proportion ! Cumulative incidence or incidence proportion Attack rate

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Rate

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Incidence rate Number of NEW cases of disease Total person - time of observation

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Incidence rate Number of NEW cases of disease 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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ABCDEABCDE 90 91 92 93 94 95 96 97 98 99 Time at risk x x 6.0 10.0 8.5 5.0 Total years at risk 35.5 -- time followed x disease onset Person-time

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ABCDEABCDE 90 91 92 93 94 95 96 97 98 99 00 Time at risk x x 6.0 10.0 8.5 5.0 Total years at risk 35.5 -- time followed x disease onset Incidence rate (IR) (Incidence density) IR = 2 / 35.5 person years = 0.056 cases / person year = 5.6 cases / 100 person years = 56 cases / 1000 person years

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Odds

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Odds (plural) Probability that an event will happen Probability that an event will not happen

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Odds WonLost Total ------------------------------------------------------------------------------------------------------------------------------------------------ EPIET facilitator team 1995-2004 14 1 15 -------------------------------------------------------------------------------------------------------------------------------------------------- Probability that an event will happen Probability that an event will not happen

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Odds WonLost Total ------------------------------------------------------------------------------------------------------------------------------------------------ EPIET facilitator team 1995-2004 14 1 15 -------------------------------------------------------------------------------------------------------------------------------------------------- Probability that an event will happen Probability that an event will not happen 14 / 15 Odds of winning = ------------- 1 / 15 = 14 : 1 = 14

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Odds of a rare event equal the risk of rare event

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The number of hepatitis cases during an outbreak Cases Non cases Population ------------------------------------------------------------------------------------ Hepatitis A 30 49,97050,000 ------------------------------------------------------------------------------------

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Odds of a rare event equal the risk of rare event The number of hepatitis cases during an outbreak Cases Non cases Population ------------------------------------------------------------------------------------ Hepatitis A 30 49,97050,000 ------------------------------------------------------------------------------------ 30 / 50,000 Odds of disease = ------------------------ = 0.006 49,970 / 50,000

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Odds of a rare event equal the risk of rare event The number of hepatitis cases during an outbreak Cases Non cases Population ------------------------------------------------------------------------------------ Hepatitis A 30 49,97050,000 ------------------------------------------------------------------------------------ 30 / 50,000 Odds of disease = ------------------------ = 0.006 49,970 / 50,000 Risk (CI) of disease = 30/50,000 = 0.006

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Summary of disease occurence Risk –Cumulative incidence or incidence proportion –Attack rate Rate –Incidence rate Odds

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Prevalence (point prevalence) Number of cases of disease at a specific time Population of interest at that time

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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 disease at a given time. Expressed as a percentage

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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 disease at a given time. Expressed as a percentage Ex: Bilharziosis in Guadeloupe in 1979: Population 350,000 Cases 96,200 Prevalence 27.6%

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References Epidemiology, an introduction. Rothman KJ. Epidemiology in Medicine. Henneckens CH, Buring JE. Modern infectious disease epidemiology. Giesecke J. Dictionary of epidemiology. Last J.

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