1. Measures of Disease Frequency COURTNEY D. LYNCH, PhD MPH ASSISTANT PROFESSOR DEPT. OF OBSTETRICS & GYNECOLOGY Courtney.Lynch@osumc.edu

2. Learning Objectives Discriminate between the various scales of disease measurement Differentiate between a rate, ratio and a proportion Define and calculate incidence Define and calculate prevalence Describe the relation between incidence and prevalence

3. Measures of disease frequency  Counts  Ratios  Proportions  Rates

4. Count  What is the problem with counts?  When is a count useful?  Example: Counts of H1N1 influenza

5. Ratio  Shows the relative size of two values  Demonstrates how many times larger (or smaller) one group is compared to another  Example: Sex ratio = boys/girls at birth = 52/48

6. Proportion  A proportion is a ratio in which the numerator is a subset of the denominator  A / A+B  Dimensionless  Example: Proportion African American (AA)= AA/AA + other races

7. Proportion vs. ratio  Medical school class with 60 males and 40 females  Proportion male:  60/100  Ratio of males/females:  60/40

8. Rate  A ratio that takes the form a/a+b during some period of time  Epidemiologic rates contain  Count of disease frequency  Size of the population at risk  Time period during which the disease occurred in the reference population  Example:  CVD mortality rate = CVD deaths/population at risk in 2012

9. Some important properties  Ratios  May or may not have units  Always > 1  Proportions  Unitless  Takes values between 0 and 1  Rates  The time period in the numerator and denominator must be the same.  A rate may or may not be a proportion but is always a ratio

10. Population dynamics  In fixed populations, membership is permanent and defined by an event/characteristic  Babies born at Wexner Medical Center in 2012  In dynamic populations, membership is transient and defined by being in or out of a ‘state’  Citizens of Franklin County

11. Measures of disease frequency  Incidence  Prevalence

12. Incidence  The frequency of the occurrence of new cases over a specified period of time  Measures the appearance of disease  Two types: cumulative incidence and incidence rate  Useful to etiologic researchers because changes in incidence suggest a change in the balance of causal factors

13. Cumulative incidence  Risk or probability of an individual getting a disease  A proportion  # of new cases of disease / # at risk at beginning of follow-up over a specified time period  Usually used in fixed populations

14. Cumulative incidence  Cumulative incidence (CI) assumes that you have followed the entire population for the entire period  But, many, if not most, populations are dynamic  Thus, CI is not an appropriate measure  Incidence density (incidence rate) takes into account differences in periods of follow-up

15. Incidence rate (density)  # of new cases / sum of disease-free person- time over a specified time period

16. Incidence rate (density) denominator  The IR denominator is the person-time at risk, defined as the sum of disease-free time in the population.  Units for denominator = person-years  The IR denominator allows for the population to be dynamic

17. Approaches to the denominator 1.Add the individual risk periods  Exact, preferred method 2.Use the average number of people in the study multiplied by the study duration  500,000 persons * 1 year = 500,000 person- years  50 persons * 50 years = 2,500 person-years 3.Use the average duration per person  100 persons * 2.5 years = 250 person-years

18. Calculating person-time 19992000200120022003 MI Lost MI 2 5 4 5 3 Exact person-time = 19 Estimate person-time 5 * 2.5 = 12.5 IR= 2/19IR= 2/12.5

19. Prevalence  The proportion of people in a population with the disease, at a specified point (or period) in time  Measures existing disease  Two types of prevalence:  Point prevalence  Period prevalence

20. Point prevalence  # of existing cases / total population at a specified point in time  A proportion

21. Period prevalence  (# of existing cases + number of cases that occur during the interval) / population at midpoint of interval (or average population size)  Also a proportion

22. Value of prevalence  Describing a health burden  Health planning/allocation of resources – treatment, hospital beds, manpower, etc.

23. Relation between prevalence and incidence  Prevalence depends on incidence and disease duration  Prevalence is similar to incidence*duration of disease [P=ID]  If a disease is of short duration, I~P  If a disease is chronic, prevalence is higher than incidence

24. Incident cases Deaths and recovery Prevalent cases Incidence and Prevalence

25. Why do epidemiologists prefer incidence?  We’re usually interested in etiology  Don’t want to vary a number of factors at the same time  Possible risk factors for disease  Factors associated with survival  The problem of birth defects

26. Summary  The burden of disease can be quantified by counts, ratios, proportions, and rates.  Incidence measures new cases  Prevalence measures existing cases  Prevalence = incidence * duration of disease

28. Thank you! If you have any questions, please contact: Courtney D. Lynch, PhD MPH Courtney.Lynch@osumc.edu