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Measures of Disease Frequency

Learn about disease frequency measures, including prevalence and incidence, and how to calculate and interpret them in different populations. Explore examples and formulas for measuring disease occurrence.

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Measures of Disease Frequency

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  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. Framingham Heart Study • Framingham Heart Study began in 1948 – 5,209

  4. 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

  5. Ex. Hypothetical Frequency of AIDS in Two Cities # New Cases Time Period Population City A 58 2005 25,000 City B 35 2005-2006 7,000

  6. Ex. Hypothetical Frequency of AIDS in Two Cities # New Cases Time Period Population City A 58 2005 25,000 City B 35 2005-2006 7,000 • 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)

  7. Three Classes of MathematicalParameters • 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

  8. 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 = .10 or 10%

  9. 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

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

  11. 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

  12. 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’

  13. 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

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

  15. 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

  16. 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 = .00201/y = 201/100,000 py(.00201 x 100,000 p/100,000 p)

  17. Review of Dimensions

  18. 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

  19. 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

  20. 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

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

  22. 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

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

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

  25. 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)

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

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

  28. 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

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

  30. 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

  31. 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

  32. Actual News • South Carolina ranks third in shark attacks — but how worried should you be? Eric Connor, The Greenville News Published 8:29 a.m. ET July 27, 2019 | Updated 11:41 a.m. ET July 29, 2019 • Let's just put it out there since it's now "Shark Week." • If you're swimming in the ocean, could you get bitten by a shark? Yes, sharks swim in the ocean. Has South Carolina had a history of attacks more than most other coastal states? Yes, South Carolina ranks third among the nation's 23 coastal states over the course of the past decade for the number sharks attacking humans. This is according to SafeWise, a security-focused professional review and comparison service, which analyzed 20 years of data logged on the Global Shark Attack File. With onset of "Shark Week" this weekend here's some perspective. The U.S. Centers for Disease Control and Prevention estimates that 91 million people swim in the ocean, lakes and rivers. Only about 44 people are attacked by sharks each year. In the past 20 years, there have been 17 fatal shark attacks. In other words, the chances of being attacked by a shark is 1 in 738 million. But still, there's something about South Carolina within that one-in-multimillion chance that stands above other states.

  33. Current News

  34. In-class exercise to practice measures of disease frequency • State which type of measure of disease frequency best describes each of the following. • cumulative incidence • incidence rate • prevalence

  35. 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.

  36. 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.

  37. 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.

  38. 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

  39. 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.

  40. 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.

  41. In-Class Exercise • 2009 2010 • Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan • 1<----------------------------------------------------------------------------------------------alive • 2<--------------------------------------dead • 3<-------------------------------------------------------------dead • 4<----dead • 5 --------------------------------------------------------------------------------------------alive • 6 -------------------------------dead • 7 -----------------------------------------------------------------------alive • 8 -------------------dead • 9 ----------------------------------------------------------alive • 10 -----------------------dead • Assume that the 990 remaining individuals in the study did not become ill or die during the year of observation.

  42. From the information and diagram given calculate the following A. Prevalence of the disease on: • January 1, 2009 • ii. July 1, 2009 • December 31, 2009 • B. Cumulative incidence during 2009:

  43. 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) D. Case Fatality “Rate” during 2009

  44. 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) D. Case Fatality “Rate” during 2009

  45. 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

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