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Chapter 3

Chapter 3. Measures of Morbidity and Mortality Used in Epidemiology Revised by Susan Bailey, Ph.D. Learning Objectives. Define and distinguish among ratios, proportions, and rates Explain the term population at risk

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Chapter 3

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  1. Chapter 3 Measures of Morbidity and Mortality Used in Epidemiology Revised by Susan Bailey, Ph.D.

  2. Learning Objectives • Define and distinguish among ratios, proportions, and rates • Explain the term population at risk • Identify and calculate commonly used rates for morbidity, mortality, and natality • State the meanings and applications of incidence rates and prevalence

  3. Learning Objectives (cont’d) • Discuss limitations of crude rates and alternative measures for crude rates • Apply direct and indirect methods to adjust rates • List situations where direct and indirect adjustment should be used

  4. Count • The simplest and most frequently performed quantitative measure in epidemiology. • Refers to the number of cases of a disease or other health phenomenon being studied. • Significant for rare diseases or symptom presentations (e.g., case of Ebola virus).

  5. Examples of Counts • Traffic fatalities in Manhattan in a 24-hour time period • College dorm students who had hepatitis • Foreign-born stomach cancer patients

  6. Ratio • The value obtained by dividing one quantity by another • The most general form has no specified relationship between numerator and denominator. • Proportions, rates, and percentages are also ratios.

  7. Number of male cases 950 Number of female cases 50 19:1 male to female = = Example of a Ratio Calculation • Sex ratio (data from textbook)

  8. Definition of Proportion • A measure that states a count relative to the size of the group. • A ratio in which the numerator is part of the denominator. • May be expressed as a percentage.

  9. Uses of Proportions • Can demonstrate the magnitude of a problem. • Example: 10 dormitory students develop hepatitis. How important is this problem? • If only 20 students live in the dorm, 50% are ill. • If 500 students live in the dorm, 2% are ill.

  10. A B Total (A + B) Number of deaths among African-American boys Number of deaths among white boys Total 1,150 3,810 4,960 Proportion = A/(A + B) x 100 = (1,150/4,960) x100 = 23.2% Example of a Proportion • Calculate the proportion of African-American male deaths among African-American and white boys aged 5 to 14 years.

  11. Rate • Definition: a ratio that consists of a numerator and a denominator and in which time forms part of the denominator. • Contains the following elements: • disease frequency • unit size of population • time period during which an event occurs

  12. Example: Number of deaths in the United States during 2003 = 2,448,288 Population of the U.S. as of July 1, 2003 = 290,810,789 2,448,288 290,810,789 Crude death rate = = 841.9 per 100,000 Example of Rate Calculation Number of deaths in a given year Reference population (during the midpoint of the year) X 100,000 Crude death rate =

  13. Definition of Prevalence • The number of existing cases of a disease or health condition in a population at some designated time.

  14. Analogy of prevalence and incidence: The water flowing down the waterfall symbolizes incidence and water collecting in the pool at the base symbolizes prevalence. Source: Robert Friis.

  15. Interpretation of Prevalence • Provides an indication of the extent of a health problem. • Example 1: Prevalence of diarrhea in a children’s camp on July 13 was 33% (point prevalence). • Example 2: prevalence of cancer in women during a specified time period (period prevalence)

  16. Uses of Prevalence • Describing the burden of a health problem in a population. • Estimating the frequency of an exposure. • Determining allocation of health resources such as facilities and personnel.

  17. Number of persons ill atpointintime Point Prevalence = Total numberin the group Example: Total number of smokers in the group = 6,234 Total number in the group 41,837 = 149.0per 1,000 or 14.9% Point Prevalence

  18. Period Prevalence = during a time period Example: Persons ever diagnosed with cancer = 2,293 Average population 41,837 = 5.5% Period Prevalence Number of persons ill Average population

  19. Definition of Incidence • The number of new cases of a disease that occur in a group during a certain time period.

  20. Incidence Rate (Cumulative Incidence) • Describes the rate of development of a disease in a group over a certain time period. • Contains three elements: • Numerator = the number of new cases. • Denominator = the population at risk. • Time = the period during which the cases occur.

  21. Applications of Incidence Data • Help in research on the etiology/causality of disease. • Used to estimate • the risk of developing a disease. • the effects of exposure to a hypothesized factor of interest.

  22. Incidence Rate Calculation (IWHS Data) Number of new cases over a time period X multiplier (e.g., 100,000) Incidence rate = Total population at risk during the same time period Number of new cases = 1,085 Population at risk = 37,105 1,085 37,105 = 0.02924/8 = 0.003655 x 100,000 Incidence rate = = 365.5 cases per 100,000 women per year

  23. x 100 (during a time period) AR = Attack Rate (AR) • Alternative form of incidence rate. • Used for diseases observed in a population for a short time period. • Not a true rate because time dimension often uncertain. • Example: Salmonella gastroenteritis outbreak • Formula: Ill___ Ill + Well

  24. Incidence Density • An incidence measure used when members of a population or study group are under observation for different lengths of time.

  25. Formulas for Incidence Density Number of new cases during the time period Total person-time of observation Incidence density = If period of observation is measured in years, formula becomes: Number of new cases during the time period Total person-years of observation Incidence density =

  26. Incidence Density, Example

  27. Interrelationship: P ID  = The prevalence (P) of a disease is proportional to the incidence rate (I) times the duration (D) of a disease. Interrelationship Between Prevalence and Incidence

  28. Interrelationship between Prevalence and Incidence • Prevalence ~Incidence when disease has a short duration (D~0) • High Prevalence could mean • true high incidence or • long duration of disease • prolonged survival • Low prevalence could mean • true low incidence or • short duration of disease • rapidlyfatal • quick/effective cure Incidence Prevalence Cure/Death

  29. Interrelationship Between Prevalence and Incidence (cont’d) • If duration of disease is short and incidence is high, prevalence becomes similar to incidence. • Short duration--cases recover rapidly or are fatal. • Example: common cold

  30. Interrelationship Between Prevalence and Incidence (cont’d) • If duration of disease is long and incidence is low, prevalence increases greatly relative to incidence. • Example: many chronic diseases

  31. Outbreak of Meningococcal Infections in a Summer School Class of 10 Students

  32. Crude birth rate Fertility rate Infant mortality rate Fetal death rate Neonatal mortality rate Postneonatal mortality rate Perinatal mortality rate Maternal mortality rate Crude Rates, Measures of Natality

  33. Crude Birth Rate Used to project population changes; it is affected by the number and age composition of women of childbearing age Number of live births within a given period Crude Birth Rate = X 1,000 population Population size at the middle of that period Sample calculation: 4,112,052 babies were born in the U.S. during 2004, when the U.S. population was 293,655,404. The birth rate was 4,112,052/293,655,404 = 14.0 per 1,000.

  34. General Fertility Rate Used for comparisons of fertility among age, racial, and socioeconomic groups. # of live births within a year General fertility rate 1,000 women aged 15-44 = X # of women aged 15-44 yrs. during the midpoint of the year Sample calculation: During 2004, there were 62,033,402 women aged 15 to 44 in the U.S. There were 4,112,052 live births. The general fertility rate was 4,112,052/62,033,402 = 66.3 per 1,000 women aged 15 to 44.

  35. Fertility rates: United States, 1950-1992. Source: Reprinted from National Center for Health Statistics, Annual Summary of Births, Marriages, Divorces and Deaths, United States, 1992, Monthly Vital Statistics Report, Vol 41, No 13, p. 3. Hyattsville, MD: National Center for Health Statistics; 1993.

  36. Infant Mortality Rate Used for international comparisons; a high rate indicates unmet health needs and poor environmental conditions. Number of infant deaths among infants aged 0-365 days during the year 1,000 live births X Infant mortality = Number of live births during the year Sample calculation: In the U.S. during 2003, there were 28,025 deaths among infants under 1 year of age and 4,089,950 live births. The infant mortality rate was (28,025/4,089,950) x 1,000 = 6.85 per 1,000 live births.

  37. Fetal Death Rate (per 1,000 live births plus fetal deaths) = Number of fetal deaths after 20 weeks or more gestation X 1,000 Number of live births + number of fetal deaths after 20 weeks or more gestation Late fetal death rate (per 1,000 live births plus late fetal deaths) = Number of fetal deaths after 28 weeks or more gestation X 1,000 Number of live births + number of fetal deaths after 28 weeks or more gestation Fetal Death Rate Used to estimate the risk of death of the fetus associated with the stages of gestation.

  38. Number of fetal deaths after 20 weeks or more gestation X 1,000 (during a year) Number of live births Fetal Death Ratio Provides a measure of fetal wastage (loss) relative to the number of live births. Fetal Death Ratio =

  39. Neonatal Mortality Rate • Reflects events happening after birth, primarily: • congenital malformations • prematurity (birth before gestation week 28) • low birth weight (birth weight less than 2,500 g)

  40. Neonatal mortality rate = Number of infant deaths under 28 days of age X 1,000 live births (during a year) Number of live births Neonatal Mortality Rate Formula

  41. Postneonatal mortality rate = Number of infant deaths from 28 days to 365 days after birth X 1,000 live births Number of live births - neonatal deaths Postneonatal Mortality Rate Reflects environmental events, control of infectious diseases, and improvement in nutrition.

  42. Infant, neonatal, and postneonatal mortality rates: United States, 1940-2003. Source: From Hoyert DL, Heron MP, Murphy SL, Kung H. Deaths: Final Data for 2003. National Vital Statistics Reports, Vol 54, No 13, p. 12. Hyattsville, MD: National Center for Health Statistics; 2006.

  43. Perinatal Mortality Rate Reflects environmental events that occur during pregnancy and after birth; it combines mortality during the prenatal and postnatal periods. Perinatal mortality rate = Number of late fetal deaths after 28 weeks or more gestation plus infant deaths within 7 days of birth X 1,000 live births and fetal deaths Number of live births + number of late fetal deaths

  44. Perinatal Mortality Ratio Number of late fetal deaths after 28 weeks or more gestation plus infant deaths within 7 days of birth Perinatal mortality ratio = X 1,000 live births Number of live births

  45. Number of deaths assigned to causes related to childbirth Number of live births Maternal Mortality Rate Reflects health care access and socioeconomic factors; it includes maternal deaths resulting from causes associated with pregnancy and puerperium (during and after childbirth). Maternal mortality rate (per 100,000 live births, including multiple births) = X 100,000 live births (during a year)

  46. Crude Rates • Use crude rates with caution when comparing disease frequencies between populations. • Observed differences in crude rates may be the result of systematic factors (e.g., sex or age distributions) within the population rather than true variation in rates.

  47. Specific Rates • Specific rates refer to a particular subgroup of the population defined in terms of race, age, sex, or single cause of death or illness.

  48. Cause-Specific Rate Cause-Specific Rate = Mortality (or frequency of a given disease) X 100,000 Population size at midpoint of time period Example: Cause-specific mortality rate (age group 25-34) due to HIV in 2003 = 1,588/39,872,598 = 4.0 per 100,000

  49. Proportional Mortality Ratio (PMR) PMR (%) = Mortality due to a specific cause during a time period X 100 Mortality due to all causes during the same time period Example: PMR (%) for HIV among the 25- to 34-year-old group = 1,588/41,300 = 3.8% Indicates relative importance of a specific cause of death; not a measure of the risk of dying of a particular cause.

  50. The 10 Leading Causes of Death, 25-34 Years, All Races, Both Sexes, U.S., 2003(Number in population = 39,872,598 Mortality for all causes = 41,300

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