1. Class session 4 measuring disease prevalence and incidence Surveillance
Epidemiology 503, Section 2

2. Last Class, Review & Summary
Considerations for outbreak investigations
Calculations used in outbreak investigations
Attack rate (identify how serious/magnitude)
Attack rate ratio (identify possible source of outbreak)
Interpretations: WHO, WHAT, WHEN, WHERE
Attack Rate = # people who became ill
# people at risk
Attack Rate Ratio = Attack rate among exposed
Attack rate among unexposed

3. Cautions from Last Class
Attack rates can be artificially elevated/lowered if we fail to consider that people eat multiple foods
Attack rate ratios must be used
DON’T just do: # sick in group A / # sick in group B

4. Ways to Measure Disease Frequency
Counts
Percentage
Proportion: Numerator is also in denominator
Ratio: No specified relationship between numerator and denominator
Rate: Relative magnitude of a change as a function over time

5. Prevalence Point Prevalence: at a given point in time
# people who ARE cases at a specified time Number of persons in the population at that time
Point Prevalence: at a given point in time
Period Prevalence: over a specified time

6. Diagnosed Diabetes Prevalence in MI
Keewenaw County:
218 cases
Prevalence = 12.5%
The prevalence of diagnosed diabetes in 2010 among adults in Keewenaw County, MI is 12.5%, or 125 per 1,000.
The prevalence of diagnosed diabetes in 2010 among adults in Washtenaw County, MI is 7.9%, or 79 per 1,000.
While there are 90 times more cases of diagnosed diabetes in Washtenaw County as compared to Keewenaw County, the prevalence is higher in Keewenaw County.
Washtenaw County:
20,309 cases
Prevalence = 7.9%

7. Incidence Proportion aka - Cumulative Incidence
risk of becoming ill during some defined time
period and in some specified population
Examples: Attack rates
UTIs among UM undergrads during 2013
Diabetes diagnoses among MA in 2011
# people who became newly ill
# people at risk

8. Incidence Proportion is most useful in a stable population
# people who became newly ill
# people at risk

9. Why Incidence Proportion not always a useful measure
Population of Interest:
Current UM undergrads
In an unstable population, we may not be able to observe all at risk individuals.
# people who developed UTI during 2013
# people at risk in 2013

10. What’s the Problem?
Students move away during the summer and we won’t know if they had a UTI during that time
# people who developed a UTI in 2013
# people at risk in 2013

11. Incidence Rate and Person Time
Number of new cases Total person-time at risk
Person Time: Total amount of time population is observed during “at risk” period; sum of all individuals follow-up time J F M A S O N D 1 L
3 months 2
8 months 3
1 month 4
12 months 5
6 months
30 months

12. Person-Time in Other Applications
My house will take 8 person-hours to paint at $40/person hours
I can hire:
1 person for 8 hours
2 people for 4 hours
8 people for 1 hour

13. What happens when you don’t have/can’t get follow-up time on everyone?
200,000
people
170,000
people
1/1/00
12/31/09
10 years

14. Average Population Size=185,000
What happens when you don’t have/can’t get follow-up time on everyone?
Study Midpoint
Average Population Size=185,000
200,000
people
170,000
people
1/1/00
12/31/09
10 years

15. Incidence Rates for Large Populations
Number of new cases Average population size X Duration
This is the estimated person time

16. Incidence Rates for Large Populations
Number of new cases Average population size X Duration
Mid-population size = (200, ,000) / 2
Time = 10 years
Person-time = 185,000 * 10 years = 1,850,000 person-years

17. Incidence vs. Prevalence
Incidence looks at NEW cases (and so gets at risk)
Prevalence looks at extent of disease at certain time (current burden)
Captures old and new cases
Related to duration of disease

18. Under “Steady State” Conditions:
Prevalence ≈ Incidence X Disease Duration

19. Interpreting Measures of Disease
Who: population or subgroup of interest
When: what time point or time period
Where: geographical location
What: what outcome/how measured
Examples:
The cumulative incidence of Kaposi sarcoma among United States AIDS patients was 1.8% during
Among hospitalized patients in England between October and November 2011, the prevalence of healthcare-associated infections was 6.4%.
The incidence rate of tuberculosis among RA patients in Taiwan was 1,411 per 100,000 person-years in 2011.

21. Thought Exercises When would prevalence ~ incidence?
When would incidence be much, much smaller than prevalence?
What could cause a increase in prevalence?

22. Surveillance Efforts
“Ongoing systematic collection, analysis, and interpretation of health data essential to the planning, implementation, and evaluation of public health practice…”
Passive Surveillance – data from reportable diseases or mandated
Active Surveillance – field visits with data collection teams

23. Today to Think about Incidence and Prevalence