1. Measures of Disease Frequency

2. Population
Group of people with a common characteristic like age, race, sex
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

3. Disease Frequency 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

4. Ex. Hypothetical Frequency of AIDS in Two Cities
# New Cases Time Period Population
City A ,000
City B ,000
Annual "rate" of AIDS
City A = 58/25,000/1yr = 232/100,000/1 yr
City B = 35/7,000/2 yrs = 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 (1 year)

5. Three Classes of Mathematical Parameters
Used to relate
number of cases of disease
size of population
time
Ratio: division of one number by another, numbers don't have to be related
Proportion: numerator is a 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

6. Prevalence
(P) Quantifies number of existing cases of disease in a population at a point or during a period of time
P = Number of existing cases of disease / Number in total population (at a point or during a period of time)
City A has 7000 people with arthritis on Jan 1st, 2009
Population of City A = 70,000
Prevalence of arthritis on Jan 1st = or 10%

7. 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

8. Two Types of Incidence Measures
Cumulative Incidence
(Abbreviated CI)
Incidence Rate
(Abbreviated IR)

9. Cumulative incidence Number of new cases of disease
Number in candidate population over a specified period of time
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 it assumes that everyone is followed for the entire time period

10. Example: Cumulative incidence of SIDS during first year of life
Population: 1,000 live births
Cases of SIDS: 10
Cumulative Incidence: 10/1,000 or 1% over one year
Note that all live births are ‘at risk’

11. 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 live births 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

12. Incidence Rate (IR)
# new cases of disease in candidate population divided by person-time of observation
This measure is a true rate because it directly integrates time into the denominator.

14. 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

15. Ex. Cohort study of the risk of breast cancer among women with hyperthyroidism
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 = /y = 201/100,000 py ( x 100,000 p/100,000 p)

16. Review of Dimensions

17. Relationship between prevalence and incidence
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

18. Conditions for equation to be true:
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

19. Figuring duration from prevalence and incidence
Lung cancer incidence rate = 45.9/100,000 py
Prevalence of lung cancer = 23/100,000
D = P / IR = 23/100,000 p / 45.9/100,000 py =0.5 years
Conclusion: Individuals with lung cancer survive 6 months from diagnosis to death

20. Uses of Prevalence and Incidence Measures
Prevalence: administration, planning, some research
Incidence: etiologic research (problems with prevalence since it combines IR and D), planning

21. 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

22. Total number of live births
Live Birth Rate
Total number of live births
1,000 people
Sometimes denominator is women of childbearing age
For one year

23. # of deaths of infants under 1 year of age
Infant Mortality Rate
# of deaths of infants under 1 year of age
1,000 live births
For one year

24. Attack Rate
# cases of disease that develop during defined period divided by # in pop. at risk at start of period (usually used for infectious disease outbreaks)

25. Case Fatality Rate
# of deaths divided by the # cases of disease for a defined period of time

26. Survival Rate
# living cases divided by # cases of disease for a defined period of time

27. Boston Globe Report on Celebrity Skiing Deaths
“Although skiing has inherent risks, it isn’t more dangerous than other common activities. The nationwide comparisons below do not reflect differing numbers of participants.”
Is this statement accurate? What are the numbers in the following charts? Incidence? Prevalence? Or something else?
Skiing deaths (one season)…………..………36
Skiing deaths (15 year average)……..………34

28. Boston Globe Report on Celebrity Skiing Deaths: Deaths in Other Sports
Parachuting…………………………………39
Scuba diving…………………………..…...104
Recreational boating………………………716
Drownings in swimming, boating and water sports ……….………………...4,500
Bicycling ……………………………………800

29. Boston Globe Report on Celebrity Skiing Deaths: Deaths in Other Sports
Accidental Deaths: How does skiing compare to other sports? Remember to consider the missing denominators when making your assessment.
Parachuting…………………………………………..……39
Scuba diving…………………………………….…..…...104
Recreational boating……………………..……………716
Swimming/Boating/water sports ……….…..4,500
Bicycling ……………………………………………………800
SOURCE: News reports, National Ski Patrol

30. Boston Globe Report on Celebrity Skiing Deaths: Other Deaths
Accidental Deaths: How does skiing compare to other activities? Remember to consider the missing denominators when making your assessment.
Auto Accidents…………………………..…42,000
Choking on food ……….……….…………...2,900
Hit by falling object ………….………………..800
Falls in Bathtub ………..…………………….4,500
Struck by lightning ………..……….…………..800
SOURCE: News reports, National Ski Patrol

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.

32. In-class exercise to practice measures of disease frequency
Percentage of infants enrolled in a day-care center who contracted impetigo during the Course of an epidemic.
Percentage of potential army recruits rejected because of poor vision.

33. In-class exercise to practice measures of disease frequency
Number of colds experienced in a year per thousand people.
Percent of deceased males who are found to have prostate cancer at autopsy.

34. In-class exercise to practice measures of disease frequency
Number of stillbirths (children born dead) per thousand live-births.
Percent of persons who have malaria that is resistant to treatment with chloroquine.

35. In-class exercise to practice measures of disease frequency
Number of newly diagnosed brain tumors in a year per 100,000 children.
Percent of live-born infants with a cardiac malformation among 100,000 live-births

36. In-Class Exercise
Part 2: An epidemiologic investigation begun on January 1st, 2009 identified a population of 1,000 individuals among whom 4 were found to have the disease under study on this date. During the year of the study, 6 additional new cases were found. Among the total of 10 cases, there were 6 deaths during the year.

37. In-Class Exercise
For the 10 cases, see the diagram on the next slide that indicates the time of case recognition, periods of observation during the study, and vital status at the time of the termination of observation. An arrow at the start of the diagram (patients 1,2,3,4) indicates that the start of disease had occurred before the study began.

38. In-Class Exercise
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan
1< alive
2< dead
3< dead
4<----dead
alive
dead
alive
dead
alive
dead
Assume that the 990 remaining individuals in the study did not become ill or die during the year of observation.

39. From the information and diagram given calculate the following
A. Prevalence of the disease on: i. January 1, 2009 Ans: Prevalence on Jan 1, 2009 – 4/1,000 ii. July 1, 2009 Ans: Prevalence on July 1, 2009 – 5/996 iii. December 31, 2009 Ans: Prevalence on December 31, 2009 – 4/994 B. Cumulative incidence during 2009: Ans: 6/996

40. From the information and diagram given calculate the following
Population Mortality Rate during 2009
Use the population size at the beginning of the year for this calculation)
Ans: Population Mortality Rate during 2009 – 6/1,000 . This is a cumulative incidence.
D. Case Fatality “Rate” during 2009
Ans: Case Fatality “Rate” during 2009 – 6/10

41. From the information and diagram given calculate the following
Population Mortality Rate during 2009
Use the population size at the beginning of the year for this calculation)
Ans: Population Mortality Rate during 2009 – 6/1,000 . This is a cumulative incidence.
D. Case Fatality “Rate” during 2009
Ans: Case Fatality “Rate” during 2009 – 6/10

42. E. Which of the above measures would be the best indicator for each of the following purposes?
Determining the effectiveness of a new treatment
Evaluating the effectiveness of a program that tries to prevent the disease
Estimating the needs for medical facilities in treating the disease

43. In-class exercise to practice measures of disease frequency
Percentage of infants enrolled in a day-care center who contracted impetigo during the Course of an epidemic.
Attack rate (cumulative incidence). Percentages are always proportions.
Percentage of potential army recruits rejected because of poor vision.
Prevalence proportion. Again, the key word is "percentage".

44. In-class exercise to practice measures of disease frequency
Number of colds experienced in a year per thousand people.
Incidence rate. (cumulative incidence is also acceptable).The events in the numerator may either be experienced multiple times by the same person (colds, myocardial infarctions) or may be unique in an individual's lifetime (first diagnosis of cancer, death).
Percent of deceased males who are found to have prostate cancer at autopsy.
Prevalence proportion.

45. In-class exercise to practice measures of disease frequency
Number of stillbirths (children born dead) per thousand live-births.
Ratio (specifically this is the "fetal death ratio". It is different from the "fetal death rate" which is proportion: number of fetal deaths/total number of births (livebirths plus fetal deaths).
Percent of persons who have malaria that is resistant to treatment with chloroquine.
Prevalence proportion.

46. In-class exercise to practice measures of disease frequency
Number of newly diagnosed brain tumors in a year per 100,000 children.
Incidence rate (cumulative incidence is also acceptable).
Percent of live-born infants with a cardiac malformation among 100,000 live-births
Prevalence proportion

47. In-Class Exercise
Part 2: An epidemiologic investigation begun on January 1st, 2009 identified a population of 1,000 individuals among whom 4 were found to have the disease under study on this date. During the year of the study, 6 additional new cases were found. Among the total of 10 cases, there were 6 deaths during the year.

48. In-Class Exercise
For the 10 cases, see the diagram on the next slide that indicates the time of case recognition, periods of observation during the study, and vital status at the time of the termination of observation. An arrow at the start of the diagram (patients 1,2,3,4) indicates that the start of disease had occurred before the study began.

49. In-Class Exercise
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan
1< alive
2< dead
3< dead
4<----dead
alive
dead
alive
dead
alive
dead
Assume that the 990 remaining individuals in the study did not become ill or die during the year of observation.

50. From the information and diagram given calculate the following
A. Prevalence of the disease on: i. January 1, 2009 Ans: Prevalence on Jan 1, 2009 – 4/1,000 ii. July 1, 2009 Ans: Prevalence on July 1, 2009 – 5/996 iii. December 31, 2009 Ans: Prevalence on December 31, 2009 – 4/994 B. Cumulative incidence during 2009: Ans: 6/996

51. From the information and diagram given calculate the following
Population Mortality Rate during 2009
Use the population size at the beginning of the year for this calculation)
Ans: Population Mortality Rate during 2009 – 6/1,000 . This is a cumulative incidence.
D. Case Fatality “Rate” during 2009
Ans: Case Fatality “Rate” during 2009 – 6/10

52. From the information and diagram given calculate the following
Population Mortality Rate during 2009
Use the population size at the beginning of the year for this calculation)
Ans: Population Mortality Rate during 2009 – 6/1,000 . This is a cumulative incidence.
D. Case Fatality “Rate” during 2009
Ans: Case Fatality “Rate” during 2009 – 6/10

53. E. Which of the above measures would be the best indicator for each of the following purposes?
Determining the effectiveness of a new treatment
Ans: Case Fatality Rate, Mortality Rate, Prevalence
Evaluating the effectiveness of a program that tries to prevent the disease
Ans: Cumulative Incidence
Estimating the needs for medical facilities in treating the disease
Ans: Prevalence, Incidence