measures of disease occurrence t ancelle / a bosman / d coulombier / a moren / p sudre / m...
TRANSCRIPT
Measures of disease occurrence
T Ancelle / A Bosman / D Coulombier / A Moren / P Sudre / M Valenciano/ P Penttinen
Epidemiology
Peter Kreidl
Objectives
• Define and use
– Risk
- Cumulative incidence (CI)
- Attack rate (AR)
- Rate
- Incidence rate (IR)
– Odds
– Prevalence
RISK
Risk
RISK
• Non-technical definition– Vague, culture-dependent– Unexpected, unusual, dangerous/negative
events
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
Cumulative Incidence (CI)(Incidence Proportion)
Number of NEW cases of disease during a period
Population at the beginning of the period
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 350,000New cases 1,250Cumulative incidence = 0.0036 per year
= 0.36 % per year= 3.6 new cases / 1000 during
a year
xx
x
x
x
x
xx disease onset
Month 1 Month12
Cumulative Incidence
Population = 12 Diseased = 7
xx
x
x
x
x
xx disease onset
Month 1 Month12
Cumulative Incidence
CI = 7/12
= 0.58 per year
= 58% per year
xx
x
x
x
x
xx 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
Attack Rate (AR)
• Cumulative incidence during an outbreakExpressed for the entire epidemic period, from the
first to the last case
• Not really a rate but a proportion!
Attack Rate (AR)
• Cumulative incidence during an outbreakExpressed 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%
RISK
• The probability that an event will occur
Proportion !
• Cumulative incidence or incidence proportion
• Attack rate
Rate
Incidence rate
Number of NEW cases of disease
Total person - time of observation
Incidence rate
Number of NEW cases of disease
Total person - time of observation
Rate
Denominator:- is a measure of time - the sum of each individual’s time at risk and free from disease
A
B
C
D
E
90 91 92 93 94 95 96 97 98 99 Time at risk
x
x
6.0
6.0
10.0
8.5
5.0
Total years at risk 35.5
-- time followedx disease onset
Person-time
A
B
C
D
E
90 91 92 93 94 95 96 97 98 99 00 Time at risk
x
x
6.0
6.0
10.0
8.5
5.0
Total years at risk 35.5
-- time followedx 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
Odds
Odds (plural)Probability that an event will happen
Probability that an event will not happen
Odds
Won Lost Total------------------------------------------------------------------------------------------------------------------------------------------------
EPIET facilitatorteam 1995-2004 14 1 15--------------------------------------------------------------------------------------------------------------------------------------------------
Probability that an event will happen
Probability that an event will not happen
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 / 15
Odds of winning = ------------- 1 / 15
= 14 : 1 = 14
Odds of a rare event equal the risk of rare event
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------------------------------------------------------------------------------------
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
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
Summary of disease occurence
• Risk
– Cumulative incidence or incidence proportion
– Attack rate
• Rate
– Incidence rate
• Odds
Prevalence (point prevalence)
Number of cases of disease at a specific time
Population of interest at that time
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
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 350,000Cases 96,200Prevalence 27.6%
References
• Epidemiology, an introduction. Rothman KJ.
• Epidemiology in Medicine. Henneckens CH, Buring JE.
• Modern infectious disease epidemiology. Giesecke J.
• Dictionary of epidemiology. Last J.