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3. Disease Frequency 3 Two types of populations, based on whether membership is permanent or transient: Fixed population: membership is permanent and defined by an event Ex. Atomic bomb survivors Dynamic population: membership is transient and defined by being in or out of a "state.� Ex. Residents of the City of Boston

4. Disease Frequency 4 Want to quantify disease occurrence in a population Measures of disease frequency should take into account: Number of individuals affected with the disease Size of source population Length of time the population was followed

5. Disease Frequency 5 Ex. Hypothetical Frequency of AIDS in Two Cities # New Cases Time Period Population City A 58 1995 25,000 City B 35 1995-96 7,000 � Annual "rate" of AIDS City A = 58/25,000/1yr = 232/100,000/1 yr City B= 35/7,000/2yrs = 17.5/7000/1 yr = 250/100,000/1 yr �Make it easy to compare rates by using same population unit (say, per 100,000 people) and time period (say, 1 year)

6. Disease Frequency 6 Ratio: division of one number by another, numbers don't have to be related Proportion: numerator is subset of denominator, often expressed as a percentage Rate: time is an intrinsic part of denominator, term is most misused Need to specify if measure represents events or people

7. Disease Frequency 7 P = Number of existing cases of disease / Number in total population (at a point or during a period of time) Ex. City A has 7000 people with arthritis on Jan 1st, 1999 Population of City A = 70,000 Prevalence of arthritis on Jan 1st = .10 or 10%

8. Disease Frequency 8 Incidence - Quantifies number of new cases of disease that develop in a population at risk during a specified time period Three key concepts: New disease events, or for diseases that can occur more than once, usually first occurrence of disease Population at risk (candidate population) - can't have disease already, should have relevant organs Time must pass for a person to move from health to disease

9. Disease Frequency 9 Cumulative Incidence (Abbreviated CI) Incidence Rate (Abbreviated IR)

10. Disease Frequency 10 Cumulative incidence = Number of new cases of disease Number in candidate population over a specified period of time

11. Disease Frequency 11 Cumulative incidence estimates the probability or risk that a person will develop disease DURING A SPECIFIED TIME. Note that the candidate population is comprised of people who are �at risk� of getting the disease Used mainly for fixed populations because its assumes that everyone is followed for the entire time period

12. Disease Frequency 12 Example: Cumulative incidence of Sudden Infant Death Syndrome during first year of life Population 1,000 livebirths Cases of SIDS 10 Cumulative Incidence 10/1,000 or 1% over one year Note that all livebirths are �at risk�

14. Disease Frequency 14 Cumulative incidence calculation assumes that you have followed the entire population for the entire follow-up period. For example, it assumes you have followed all of the livebirths for one year or until SIDS occurred. Often you can't follow everyone for entire time period In a dynamic population, individuals enter population over time, become lost, etc. So length of follow-up is not uniform for all Incidence rates do not make assumption of complete follow-up

15. Disease Frequency 15 Incidence rate (IR) = # new cases of disease person-time of observation in candidate population This measure is a true rate because it directly integrates time into the denominator.

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17. Disease Frequency 17 Some Ways to Accrue 100PY 100 people followed 1 year each = 100 py 10 people followed 10 years each= 100 py 50 people followed 1 year plus 25 people followed 2 years = 100 py Time unit for person-time = year, month or day Person-time = person-year, person-month, person-day

18. Disease Frequency 18 Followed 1,762 women ---> 30,324 py Average of 17 years of follow-up per woman Ascertained 61 cases of breast cancer Incidence rate = 61/30,324 py = .00201/y = 201/100,000 py (.00201 x 100,000 p/100,000 p)

19. Disease Frequency 19 Review of Dimensions

20. Disease Frequency 20 P / (1-P) = IR x D Prevalence depends on incidence rate and duration of disease (duration lasts from onset of disease to its termination) If incidence is low but duration is long - prevalence is relatively high If incidence is high but duration is short - prevalence is relatively low

21. Disease Frequency 21 Steady state IR constant Distribution of durations constant Note that if the prevalence of disease is low (less than 10%), the equation simplifies to P = IR x D

22. Disease Frequency 22 Figuring duration from prevalence and incidence

23. Disease Frequency 23 Prevalence: administration, planning, some research Incidence: etiologic research (problems with prevalence since it combines IR and D), planning

24. Disease Frequency 24 Common measures of disease frequency for public health Crude death (mortality) rate: Total number of deaths from all causes 1,000 people For one year (also cause-specific, age-specific, race-specific death rate)

25. Common measures of disease frequency for public health Live-birth rate: total number of livebirths For one year 1,000 people (sometimes women of childbearing age) � Infant mortality rate: # deaths of infants under 1 year of age For one year 1,000 live-births

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29. Disease Frequency 29 Accidental Deaths: How does skiing compare to other sports? Remember to consider the missing denominators when making your assessment.

30. Accidental Deaths: How does skiing compare to other activities? Remember to consider the missing denominators when making your assessment.

31. Disease Frequency 31 In-class exercise to practice measures of disease frequency�Part 1: State which type of mathematical parameter (ratio/proportion/rate) and which type of measure of disease frequency (cumulative incidence/incidence rate/prevalence) best describes each of the following. 1. Percentage of infants enrolled in a day-care center who contracted impetigo during the course of an epidemic.

32. Disease Frequency 32 State which type of mathematical parameter (ratio/proportion/rate) and which type of measure of disease frequency (cumulative incidence/incidence rate/prevalence) best describes each of the following.

33. State which type of mathematical parameter (ratio/proportion/rate) and which type of measure of disease frequency (cumulative incidence/incidence rate/prevalence) best describes each of the following.3. Number of colds experienced in a year per thousand people.

34. Disease Frequency 34 State which type of mathematical parameter (ratio/proportion/rate) and which type of measure of disease frequency (cumulative incidence/incidence rate/prevalence) best describes each of the following. 4. Percent of deceased males who are found to have prostate cancer at autopsy.

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43. Disease Frequency 43 E. Which of the above measures would be the best indicator for each of the following purposes: i. Determining the effectiveness of a new treatment ii. Evaluating the effectiveness of a program that tries to prevent the disease iii. Estimating the needs for medical facilities in treating the disease