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

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

3. RISK

4. Risk

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

6. 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

8. 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

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

10. 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

11. 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

12. 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%

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

15. Rate

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

17. 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

18. 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

19. 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

20. Odds

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

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

23. 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

24. Odds of a rare event equal the risk of rare event

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

26. 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

27. 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

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

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

30. 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

31. 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%

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