Rapport Proportion Taux

1
Measures of disease occurrence
Peter Kreidl
T Ancelle / A Bosman / D Coulombier / A Moren / P
Sudre / M Valenciano/ P Penttinen
2
Objectives

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

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RISK
4
Risk
5
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

7
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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Cumulative Incidence
Population 12 Diseased 7
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Cumulative Incidence
CI 7/12 0.58 per year 58 per year
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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!

13
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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Person-time
90 91 92 93 94 95 96 97 98 99
Time at risk
A B C D E
6.0 6.0 10.0 8.5 5.0
x
x
Total years at risk
35.5 -- time followed x disease
onset
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Incidence rate (IR)(Incidence density)
90 91 92 93 94 95 96 97 98 99 00
Time at risk
IR 2 / 35.5 person years 0.056 cases
/ person year 5.6 cases / 100 person years
56 cases / 1000 person years
A B C D E
6.0 6.0 10.0 8.5 5.0
x
x
Total years at risk
35.5 -- time followed x disease
onset
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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
Probability that an event will happen
Probability that an event will not happen

• Won Lost Total
• --------------------------------------------------
  --------------------------------------------------
  --------------------------------------------
• EPIET facilitator
• team 1995-2004 14 1 15
• --------------------------------------------------
  --------------------------------------------------
  ----------------------------------------------

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Odds
Probability that an event will happen
Probability that an event will not happen

• Won Lost Total
• --------------------------------------------------
  --------------------------------------------------
  --------------------------------------------
• EPIET facilitator
• team 1995-2004 14 1 15
• --------------------------------------------------
  --------------------------------------------------
  ----------------------------------------------

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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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 49,970 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 49,970 50,000
• --------------------------------------------------
  ----------------------------------

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 49,970 50,000
• --------------------------------------------------
  ----------------------------------

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.