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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.