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Slide 53

The proportion of infants who are born alive with a defect of the ventricular septum of the heart is a prevalence or incidence?. Slide 53. Prevalence proportion is used more in public health than in causal research

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Slide 53

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  1. The proportion of infants who are born alive with a defect of the ventricular septum of the heart is a prevalence or incidence? Slide 53

  2. Prevalence proportion is used more in public health than in causal research However, there are research areas in which prevalence measures are used more commonly than incidence measures, e.g., birth defects (when we describe occurrence of congenital malformations among live-born infants in terms of proportion of these infants who have a malformation), diseases that are difficult to define onset (diabetes, hypertension)

  3. PREVALENCE MEASURES Slide 44 – Slide 53

  4. DISEASE FREQUENCY • Both incidence proportion and incidence rate are measures that assess frequency of disease onset • Prevalence proportion is a measure of disease status

  5. PREVALENCE MEASURES Prevalence is the frequency of existing cases

  6. Measle Infected Baby Immuned Baby Measles in a cohort of unimmunized infants Healthy Baby Modified from Ralph Frerichs, UCLA (http://www.ph.ucla.edu/epi/200/epi200_01.html)

  7. 1 2 3 4 5 6 7 Weeks Five babies observed for 7 wks for measles

  8. 1 2 3 4 5 6 7 Weeks For point prevalence, watch the blue bar

  9. Prevalence is calculated by: Number of people with the disease or condition at a specific time P = Total population at a specific time

  10. 1 2 3 4 5 6 7 Weeks How many babies had measles at week 2? 1

  11. 1 2 3 4 5 6 7 Weeks How many total population at week 2? 5

  12. 1 existing case in a population of 5 babies 1 Prevalence = = 0.2 = 20% 5 • Point prevalence is: The proportion of the population affected by a disease at a specific point in time

  13. PERIOD PREVALENCE • Period prevalence is calculated by: Number of incident and prevalent cases identified during a given period • P = Size of the total population during the period

  14. 1 2 3 4 5 6 7 Weeks For period prevalence, watch the green bar

  15. 1 2 3 4 5 6 7 Weeks How many babies had measles during weeks 2-4? 3

  16. 1 2 3 4 5 6 7 Weeks How many baby population during week 2-4? 5

  17. 3 existing case in a population of 5 babies 3 Period revalence = = 0.6 = 60% 5 • Period prevalence is: The proportion of the population affected by a disease anytime during a given period

  18. INTERPRETATION OF PREVALENCE • Because prevalence reflects both incidence rate and disease duration, it is not as useful as incidence for studying causes of disease. • It is useful for measuring disease burden on a population, especially if those who have the disease require specific medical attention.

  19. RELATIONSHIP BETWEEN PREVALENCE AND INCIDENCE Prevalence is less useful than incidence in etiologic studies, because it is a function of incidence rate ( ) and duration of disease ( ) Assumption: prevalence, incidence rate and mortality rate remain constant over time, no in- and out-migration I

  20. RELATIONSHIP BETWEEN PREVALENCE AND INCIDENCE If the disease is rare, = mean duration of disease Assumption: prevalence, incidence rate and mortality rate remain constant over time, no in- and out-migration

  21. Four million people are dying from tobacco per year! Slide 56 – Slide 65

  22. Prevalence - example • You are the new hospital epidemiologist in a busy tertiary care hospital in Hanoi • On your first day at work the hospital director asks you to review the hospital data and tell the staff about Ventilator Associated Pneumonia (VAP)

  23. Prevalence - example • You notice that there are 3 cases of VAP in the hospital today of 498 hospitalized patients

  24. Prevalence - example • You notice that there are 3 cases of VAP in the hospital today of 498 hospitalized patients • At the morning staff meeting you announce that the point prevalence of VAP is 0.6% • The director pulls you aside later and asks you to do some more work

  25. Incidence - example • You find 24 cases of VAP over the last year at your hospital and a medical records staff person tells you that 148,637 total patients were given care in the last year and 32,648 patients were hospitalized

  26. Incidence - example • You find 24 cases of VAP over the last year at your hospital and a medical records staff person tells you that 148,637 total patients were given care in the last year and 32,648 patients were hospitalized • At the morning staff meeting you announce that the incidence rate of VAP is .02% per year

  27. Incidence - example • You find 24 cases of VAP over the last year at your hospital and a medical records staff person tells you that 148,637 total patients were given care in the last year and 32,648 patients were hospitalized • At the morning staff meeting you announce that the incidence rate of VAP is .02%/year • The director pulls you aside later and asks you to do some more work

  28. Incidence - example • You decide to become friends with the staff in medical records • Working with a staff person you determine that 272 persons were on a ventilator during the last year

  29. Incidence - example • You decide to become friends with the staff in medical records • Working with a staff person you determine that 272 persons were on a ventilator during the last year • At the morning meeting you announce that the incidence proportion (risk) of VAP is 8.8% • The staff are confused and ask why the numbers keep changing

  30. Incidence - example • The staff asks if this is the most useful information you can provide regarding risk of VAP

  31. Incidence - example • The staff asks if this is the most useful information you can provide regarding risk of VAP • You say yes

  32. Incidence - example • The staff asks if this is the most useful information you can provide regarding risk of VAP • You say yes • The director says no and asks you to do some more work

  33. Incidence - example • You stay up very late with your friend from medical records • Together, you determine that the 272 patients had a grand total of 1,105 days on the ventilator

  34. Incidence - example • You stay up very late with your friend from medical records • Together, you determine that the 272 patients had a grand total of 1,105 days on the ventilator • At the morning staff meeting, you announce that the incidence rate of VAP is 2.17 per 100 ventilator days • The staff cheers

  35. Incidence - example • The director asks the staff to stop cheering and asks you to do some more work • The director also says that you may show some promise

  36. Incidence - example • You stay up all night with your friend from medical records • Together, you determine that of the 1,105 days on the ventilator you previously counted, 221 occurred after VAP

  37. Incidence - example • You stay up all night with your friend from medical records • Together, you determine that of the 1,105 days on the ventilator you previously counted, 221 occurred after VAP • At the morning staff meeting, you announce that the correct incidence rate of VAP is 2.71 per 100 ventilator days • The staff cheers again, and asks that you attend a different meeting tomorrow morning

  38. Incidence - example • The director asks the staff to stop cheering and asks you to stratify by hospital day and calculate incidence of VAP by strata of days on ventilator • You ask the director if he will support your attendance in a month long intro to field epi course in Bangkok • The staff cheers, except for your friend in medical records

  39. Incidence - example • The director agrees and says he has a great new project in mind regarding the occurrence of diarrhea in patients while on antibiotic therapy • He asks you to present initial results at the first meeting after you get back to the hospital • The staff does not cheer

  40. CASE FATALITY • Case fatality risk (CFR) is defined as: The probability of a case dying from the disease during a given period • CFR is calculated by: Number of deaths from a disease during a specified period after disease occurrence CFR = Number of incident cases of the disease during that period

  41. 11. Compute case fatality risk: should fine better data

  42. BASIC CONCEPTS OF MEASUREMENT • Ratio: a ratio expresses the relationship between two numbers in the form x : y • Proportion: a proportion is a fraction in which all elements of the numerator are included in the denominator • Rate: a rate is an instantaneous change in one quantity per unit of time

  43. INFANT MORTALITY RATIO Number of deaths in a year of children less than 1 year of age = Number of live births in the same year • Perinatal mortality: 28 wks gestation 1 wk of life • Neonatal mortality: 1st month of life • Post neonatal mortality: 1 month 1 year

  44. 90% 79% 67% 54% 36% 18% KAPLAN-MEIER CURVE 1 year survival = ? Cumulative survival 2 year survival = ? median survial = ? 0 2 4 6 8 10 12 14 16 18 20 22 24 Time (months) Modified from fig. 2-1 in M. Szklo, F. Nieto. Epidemiology Beyond the Basics. Maryland: Aspen Publishers; 2000

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