1. Epidemiology Kept Simple
Chapter 6: Incidence & Prevalence
4/17/2017
Epidemiology Kept Simple
Chapter 6
Incidence and Prevalence
(c) B. Gerstman
Chapter 6
Epidemiology 1

2. Measures of Disease Occurrence
Incidence proportion (risk)
Incidence rate (density)
Prevalence
All three are loosely called “rates”
(But only the second is a true rate)
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Chapter 6

3. Types of Populations
We measure disease occurrence in two types of populations:
Closed populations  “cohorts”
Open populations

4. Closed Population ≡ Cohort
Cohort word origin (Latin cohors) basic tactical unit of a Roman legion
Epi cohort ≡ a group of individuals followed over time
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Chapter 6

5. Open Populations Inflow (immigration, births)
Outflow (emigration, death)
An open population in “steady state” (constant size) is said to be stationary
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Chapter 6

6. Numerators & Denominators
“Rates” are composed of numerators and denominators
Numerator  case count
Incidence count  onsets
Prevalence count  old + new cases
Denominators  reflection of population size
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Chapter 6

7. Denominators Denominators: reflection of population size
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Chapter 6

8. Incidence Proportion (IP)
Can be calculated only in cohorts
Synonyms: risk, cumulative incidence, attack rate
Interpretation: average risk
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Chapter 6

9. 10-year average risk is .011 or 1.1%.
Example
Objective: estimate risk of uterine cancer
Recruit cohort of 1000 women
100 had hysterectomies, leaving 900 at risk
Follow at risk individuals for 10 years
Observe 10 onsets of uterine cancer
10-year average risk is .011 or 1.1%.
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Chapter 6

10. Incidence Rate (IR) Synonyms: incidence density, person-time rate
Interpretation A: “Speed” at which events occur
Interpretation B: When disease is rare: rate per person-year ≈ one-year risk
Calculated differently in closed and open populations
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Chapter 6

11. Rate is .00111 per year or 11.1 per 10,000 years
Example
Objective: estimate rate of uterine cancer
Recruit cohort of 1000 women
100 had hysterectomies, leaving 900 at risk
Follow at risk individuals for 10 years
Observe 10 onsets of uterine cancer
Rate is per year or 11.1 per 10,000 years
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Chapter 6 11

12. Individual follow-up over time
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Chapter 6 12

13. Mortality Rate & Life Expectancy
In stationary populations, and in cohorts with complete follow-up, the mortality rate is the reciprocal of life expectancy (and vice versa).
Example: for a mortality rate of per year
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Chapter 6

14. Mortality Rate and Life Expectancy
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Chapter 6 14

15. Incidence Rate, Open Population
Example: 2,391,630 deaths
In 1999 (one year)
Population size = 272,705,815
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Chapter 6

16. Prevalence Point prevalence ≡ prevalence at a particular point in time
Period prevalence ≡ prevalence over a period of time
Interpretation A: proportion with condition
Interpretation B: probability a person selected at random will have the condition
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Chapter 6

17. Example: Prevalence of hysterectomy
Recruit 1000 women
Ascertain: 100 with hysterectomies
Prevalence in sample is 10%
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Chapter 6

18. Dynamics of Prevalence Cistern Analogy (p. 185)
Ways to increase prevalence
Increase incidence  increase inflow
Increase average duration of disease  decreased outflow
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19. Relation Between Incidence and Prevalence
When disease rare & population stationary
Example:
Incidence rate = 0.01 / year
Average duration of the illness = 2 years.
Prevalence ≈ 0.01 / year × 2 years = 0.02
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Chapter 6