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Epidemiologic measures: Incidence & prevalence

Principles of Epidemiology for Public Health (EPID600). Epidemiologic measures: Incidence & prevalence. Victor J. Schoenbach, PhD home page Department of Epidemiology Gillings School of Global Public Health University of North Carolina at Chapel Hill www.unc.edu/epid600/. Famous last words.

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Epidemiologic measures: Incidence & prevalence

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  1. Principles of Epidemiology for Public Health (EPID600) Epidemiologic measures: Incidence & prevalence Victor J. Schoenbach, PhD home page Department of EpidemiologyGillings School of Global Public HealthUniversity of North Carolina at Chapel Hill www.unc.edu/epid600/ Incidence and prevalence

  2. Famous last words Quotations that demonstrate the value of humility about predicting the future(authenticity not established) Courtesy of Suzanne Cloutier, 11/17/1998 Incidence and prevalence

  3. “Louis Pasteur's theory of germs is ridiculous fiction.” - Pierre Pachet, Professor of Physiology at Toulouse, 1872 FAMOUS LAST WORDS: quotations that demonstrate the value of humility in predicting the future Incidence and prevalence

  4. “This `telephone´ has too many shortcomings to be seriously considered as a means of communication. The device is inherently of no value to us.” - Western Union internal memo, 1876 (Source: 2000 National Ernst & Young Entrepreneur of the Year Awards special insert in USA Today, 2/11/2000, p9B) FAMOUS LAST WORDS: quotations that demonstrate the value of humility in predicting the future Incidence and prevalence

  5. “Everything that can be invented has been invented.” - Charles H. Duell, Commissioner, US Patent Office,1899 FAMOUS LAST WORDS: quotations that demonstrate the value of humility in predicting the future Incidence and prevalence

  6. “The wireless music box has no imaginable commercial value. Who would pay for a message sent to nobody in particular?” - David Sarnoff's associates in response to his urgings for investment in radio in the 1920s. FAMOUS LAST WORDS: quotations that demonstrate the value of humility in predicting the future Incidence and prevalence

  7. “My thesis in this lecture is that macroeconomics . . . has succeeded: Its central problem of depression-prevention has been solved, for all practical purposes, and has in fact been solved for many decades.” - Robert E. Lucas, Jr., American Economics Association Presidential Address, January 10, 2003http://home.uchicago.edu/~sogrodow/homepage/paddress03.pdf FAMOUS LAST WORDS: quotations that demonstrate the value of humility in predicting the future Incidence and prevalence

  8. The population perspective requires measuring disease in populations • Science is built on classification and measurement. • Reality is infinitely detailed, infinitely complex. • Classification and measurement seek to capture the essential attributes. Incidence and prevalence

  9. Deriving meaning from stimuli Vase or faces? Which line is longer? Incidence and prevalence

  10. Measurement “captures” the phenomenon • Classification and measurement are based on: • Objective of the classification • Conceptual model (understanding of the phenomenon) • Availability of data (technology) Incidence and prevalence

  11. An example population (N=200)         O O O O O O O Incidence and prevalence

  12. How can we quantify disease in populations?         O Incidence and prevalence

  13. How can we quantify disease in populations?         O O Incidence and prevalence

  14. How can we quantify disease in populations?         O O O Incidence and prevalence

  15. How can we quantify disease in populations? O O         O O O Incidence and prevalence

  16. How can we quantify disease in populations? O O         O O O O Incidence and prevalence

  17. How can we quantify the frequency? O O         O O O O Incidence and prevalence

  18. Rate of occurrence of new casesper unit time (e.g., 1 per month) O O         O O O O Incidence and prevalence

  19. 1 new case in month 1         O Incidence and prevalence

  20. 1 new case in month 2         O O Incidence and prevalence

  21. 1 new case in month 3, for a total of 3 cases         O O O Incidence and prevalence

  22. 2 new cases in month 4 O O         O O O Incidence and prevalence

  23. 1 new case in month 5 (total=6) O O         O O O O Incidence and prevalence

  24. 1 case in month 6 O O         O O O O O Incidence and prevalence

  25. 1 new case in month 7 O O         O O O O O O Incidence and prevalence

  26. 2 new cases in month 8 O O         O O O O O O O O Incidence and prevalence

  27. 2 cases in month 9 O O         O O O O O O O O O O Incidence and prevalence

  28. Rate of occurrence of new cases during 9 months: 1 case/month to 2 cases/month O O         O O O O O O O O O O Incidence and prevalence

  29. Number of cases depends on length of interval Divide by length of time interval, so can compare across intervals Number of new casesRate of new cases = ––––––––––––––––– Time interval = 12 cases / 9 months = 1.33 cases / month Incidence and prevalence

  30. Number of cases depends on population size So, divide by population and time: Number of new casesIncidence rate = –––––––––––––––––– Population-time Incidence and prevalence

  31. How to estimate population-time? Population at risk: the people eligible to become a case and to be counted as one. In this example that population declines as each case occurs. So estimate population-time as . . . Incidence and prevalence

  32. Population-time = Method 1: Add up the time that each person is at risk Method 2: Add up the population at risk during each time segment Method 3: Multiply the average size of the population at risk by the length of the time interval Incidence and prevalence

  33. Estimating population-time - method 2 Total population-time over 9 months = 200 + 199 + 198 + 197 + 195 + 194 + 193 + 192 + 190 = 1,758 person-months = 146.5 person-years However, cases are not at risk for a full month. Incidence and prevalence

  34. Estimating population-time - method 2 - better Total population-time over 9 months = 199.5 + 198.5 + 197.5 + 196 + 194.5 + 193.5 + 192.5 + 191 + 189 = 1,752 person-months = 146 person-years assuming that cases develop, on average, in the middle of the month Incidence and prevalence

  35. Estimating population-time - method 3 Average size of the population at risk during the 9 months = 195.3 (1,758 / 9) or approximately: (200 + 188) /2 = 194 Population-time = 195.3 x 9 months or (approximately) 194 x 9 months = 1,746 person-months = 145.5 person-years Incidence and prevalence

  36. Equivalent to - method 3 Take initial size of population at risk and reduce it for time the people were not at risk due to acquiring the disease: 200 - 12/2 = 194 (approximately) Population-time = 194 x 9 months = 1,746 person-months = 145.5 person-years Incidence and prevalence

  37. Incidence rate (“incidence density”) Number of new cases –––––––––––––––––––––––––––––––Avg population at risk × Time interval Number of new cases = –––––––––––––––––––– Population-time Incidence and prevalence

  38. What proportion of the population at risk are affected after 5 months?         O O O O O O O Incidence and prevalence

  39. What proportion of the population is affected after 1 month? (1/200)         O Incidence and prevalence

  40. What proportion of the population is affected after 2 months? (2/200)         O O Incidence and prevalence

  41. What proportion of the population is affected after 3 months? (3/200)         O O O Incidence and prevalence

  42. What proportion of the population is affected after 4 months? (5/200) O O         O O O Incidence and prevalence

  43. 6 / 200 = 0.03 = 3% = 30 / 1,000 in 5 months O O         O O O O Incidence and prevalence

  44. Incidence proportion (“cumulative incidence”) Number of new cases5-month CI = ––––––––––––––––––– Population at risk Incidence proportion estimates risk. Incidence and prevalence

  45. Incidence rate versus incidence proportion • Incidence rate measures how rapidly cases are occurring. • Incidence proportion is cumulative. • When care only about the “bottom line” (i.e., what has happened by the end of given period): incidence proportion (CI). Incidence and prevalence

  46. Incidence rate versus incidence proportion • If risk period is long (e.g., cancer), we usually observe only a portion. • To compare results from studies with different length of follow-up, use incidence rate (IR) • If risk period is short, we usually observe all of it and can use incidence proportion. Incidence and prevalence

  47. Incidence rate versus incidence proportion(rare disease, IR = 0.005 / month) (see spreadsheet at epidemiolog.net/studymat/) Incidence and prevalence

  48. Incidence rate versus incidence proportion (common disease, IR = 0.1 / month) Incidence and prevalence

  49. Case fatality rate • “Case fatality rate” (but it’s really a proportion) • = proportion of cases who die (in a specified time interval) • Like a “cumulative incidence of death” in cases[ “incidence rate of death” in cases = “termination rate” = 1/(average survival time)] Incidence and prevalence

  50. Mortality rate Number of deathsMortality rate = –––––––––––––––––––––––––––– Population at risk × Time interval Number of deathsAnnual mortality rate = –––––––––––––––––––––– Mid-year population (x 1 yr) Incidence and prevalence

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