23. Prevalence Ratio males/women=2/8 Point Prevalence: Mar 1 =3/10 Proportion of women=2/10=2% Period Prevalence Mar-Jul =7/10 3 P-mo 3 P-mo 2 P-mo 4 P-mo 4 P-mo 2 P-mo D D D D 0 1 2 3 4 5 6 7 8 9 10 Jan Feb Mar Apr May Jun Jul
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34. Incidence Incidence rate (IR) Incidence density (ID) Rate (speed) For open population Cumulative incidence (CI) Incidence proportion (IP) Risk (probability) For closed population 2. Incidence — Types
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36. Risk CI assumes that entire population at risk followed up for specified time period x x x x x x x x disease onset lost to followup Month 1 Month12 CI = 7/12 per year = 0.58 per year 2. Incidence — Cumulative Incidence (CI)
43. Limitation of CI Risk May under- or over-estimate the risk when a high loss rate x x x x x x x x disease onset lost to followup Month 1 Month12 CI = 7/12 per year = 0.58 per year 2. Incidence — Cumulative Incidence
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45. Incidence rate Rate Instantaneous concept (like speed) Denominator: - is a measure of time - the sum of each individual’s time at risk and free from disease - IR = No. of new cases/ total time = news cases / unit time
46. A B C D E 90 91 92 93 94 95 96 97 98 99 00 Time at risk x x 6.0 6.0 11.0 9.5 5.0 Total years at risk 37.5 -- time followed x disease onset lost-to-followup ID = 2 / 37.5 person- years = 0.053 person-year 2. Incidence — Incidence Rate
47. Open population Population at risk births deaths cases Immigration Emigration/losses time 2. Incidence — Incidence Rate
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51. Relationship of CI and IR 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 1 3 5 7 9 11 13 15 17 19 21 Age Cumulative incidence Incidence rate Cumulative incidence 1-14 years Cumulative incidence 1-7 years
52. Relationship of CI and IR time Cumulative incidence 0.2 0.4 0.6 0.8 1.0 Slope= Incidence rate If IR is constant, CI = IR time
53. In general: Risk estimates derived from IR and CI calculations will be similar when: • Follow-up loss is minimal • The disease of interest o ccurs infrequently. CI is most useful if interest centers on the average risk of an individual will become ill over a specified period of time. IR is preferred if interest centers on how fast the new cases are occurring in the population. Comparison of IR and CI
60. Prevalence and incidence Ratio males/women=2/8 Point Prevalence: Mar 1 =3/10 Proportion of women=2/10=2% Period Prevalence Mar-Jul =7/10 Incidence Proportion Mar-Jul =4/7 Incidence density Mar-Jul = 4/18 person-month 3 P-mo 3 P-mo 2 P-mo 4 P-mo 4 P-mo 2 P-mo D D D D 0 1 2 3 4 5 6 7 8 9 10 Jan Feb Mar Apr May Jun Jul
61. Prevalence vs incidence Point prevalence Period prevalence CI IR Numerator Existing cases (old and new) Existing and new cases New cases New cases Denominator Initial Pop. Mid-point Pop. Initial or Midyear Pop. Person-time Time One point A period A period A period Unit None None None No./PT Type proportion proportion proportion rate Example question Do you currently have asthma? Have you had asthma during the last year? Have you ever had asthma last year? Study types Cross-sectional studies surveillance closed Cohort studies Open Cohort studies
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65. Discussion Question Why is incidence preferred over prevalence when studying the etiology of disease?
81. Death, by broad cause group in 2000 Injuries (9.1%) Noncommunicable conditions (59. 0 %) Communicable diseases, maternal and perinatal conditions and nutritional deficiencies (31. 9 %) Total deaths: 55,694,000 Source: WHO, World Health Report 200 1
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