Measures of disease occurrence T Ancelle / A Bosman / D Coulombier / A Moren / P Sudre / M Valenciano/ P Penttinen 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 350,000 New cases 1,250 Cumulative incidence= per year = 0.36 % per year = 3.6 new cases / 1000 during a year

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

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

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

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!

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%

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 individuals time at risk and free from disease

ABCDEABCDE Time at risk x x Total years at risk time followed x disease onset Person-time

ABCDEABCDE Time at risk x x Total years at risk time followed x disease onset Incidence rate (IR) (Incidence density) IR = 2 / 35.5 person years = 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 WonLost Total EPIET facilitator team Probability that an event will happen Probability that an event will not happen

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

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,

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, / 50,000 Odds of disease = = ,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 49,97050, / 50,000 Odds of disease = = ,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 350,000 Cases 96,200 Prevalence 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.