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Measures of disease occurrence

Measures of disease occurrence. Peter Kreidl. T Ancelle / A Bosman / D Coulombier / A Moren / P Sudre / M Valenciano/ P Penttinen. Objectives. D efine and use Risk - Cumulative incidence (CI) - Attack rate (AR) - Rate - Incidence rate (IR) Odds Prevalence. RISK. Risk. RISK.

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Measures of disease occurrence

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  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

  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 NEWcases of disease during a period Population at the beginning of the period

  8. Cumulative Incidence (CI)(Incidence Proportion) Number of NEWcases 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 disease onset x Month12 Month 1 Cumulative Incidence Population = 12 Diseased = 7

  10. x x x x x x x disease onset x Month12 Month 1 Cumulative Incidence CI = 7/12 = 0.58 per year = 58% per year

  11. x x x x x x x disease onset x Month12 Month 1 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 NEWcases of disease Total person - time of observation

  17. Incidence rate Number of NEWcases 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

  18. 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

  19. 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

  20. Odds

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

  22. Odds Probability that an event will happen Probability that an event will not happen Won Lost Total ------------------------------------------------------------------------------------------------------------------------------------------------ EPIET facilitator team 1995-200414 1 15 --------------------------------------------------------------------------------------------------------------------------------------------------

  23. Odds Probability that an event will happen Probability that an event will not happen Won Lost Total ------------------------------------------------------------------------------------------------------------------------------------------------ EPIET facilitator team 1995-200414 1 15 -------------------------------------------------------------------------------------------------------------------------------------------------- 14 / 15 Odds of winning = ------------- 1 / 15 = 14 : 1 = 14

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

  25. 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 ------------------------------------------------------------------------------------

  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 49,970 50,000 ------------------------------------------------------------------------------------ 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 49,970 50,000 ------------------------------------------------------------------------------------ 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.

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