1. Chapter 3: Measures of Morbidity and Mortality Used in Epidemiology
PU 520
Unit 3
Seminar
(PART I)

2. Define and distinguish among ratios, proportions, and rates
Outcomes
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
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3. Apply direct and indirect methods to adjust rates
Outcomes (cont’d)
Discuss limitations of crude rates and alternative measures of crude rates
Apply direct and indirect methods to adjust rates
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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).
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5. Traffic fatalities in Manhattan in a 24-hour time period
Examples of Counts
Traffic fatalities in Manhattan in a 24-hour time period
College dorm students who had hepatitis
Foreign-born stomach cancer patients
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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.
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7. Example of a Ratio Calculation
Sex ratio (data from textbook)
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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.
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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.
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10. Example of a Proportion
Calculate the proportion of African-American male deaths among African-American and white boys aged 5 to 14 years.
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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
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12. Example of Rate Calculation
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13. Definition of Prevalence
The number of existing cases of a disease or health condition in a population at some designated time.
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14. 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)
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15. 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.
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16. Point Prevalence
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17. Number of persons ill Average population
Period Prevalence
Number of persons ill Average population
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18. Definition of Incidence
The number of new cases of a disease that occur in a group during a certain time period.
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19. 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.
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20. 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.
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21. Incidence Rate Calculation (IWHS Data)
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22. 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:
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23. Incidence Density
An incidence measure used when members of a population or study group are under observation for different lengths of time.
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24. Formulas for Incidence Density
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25. Incidence Density, Example
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26. Interrelationship Between Prevalence and Incidence
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27. 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
If duration of disease is long and incidence is low, prevalence increases greatly relative to incidence.
Example: many chronic diseases
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28. Outbreak of Meningococcal Infections in a Summer School Class of 10 Students
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29. Crude Rates, Measures of Natality
Crude birth rate
Fertility rate
Infant mortality rate
Fetal death rate
Neonatal mortality rate
Postneonatal mortality rate
Perinatal mortality rate
Maternal mortality rate
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30. Crude Birth Rate
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31. General Fertility Rate
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32. Fertility rates: United States, 1950-1992
Fertility rates: United States, 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.
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33. Infant Mortality Rate
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34. Fetal Death Rate
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35. Fetal Death Ratio
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36. 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)
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37. Neonatal Mortality Rate Formula
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38. Postneonatal Mortality Rate
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39. © 2009 Jones and Bartlett Publishers, LLC (www.jbpub.com)
Infant, neonatal, and postneonatal mortality rates: United States, Source: From Hoyert DL, Heron MP, Murphy SL, Kung H. Deaths: Final Data for National Vital Statistics Reports, Vol 54, No 13, p. 12. Hyattsville, MD: National Center for Health Statistics; 2006.
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40. Perinatal Mortality Rate
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41. Perinatal Mortality Ratio
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42. Maternal Mortality Rate
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43. 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.
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44. 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.
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45. Cause-Specific Rate
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46. Proportional Mortality Ratio (PMR)
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47. The 10 Leading Causes of Death, 25-34 Years, All Races, Both Sexes, U
The 10 Leading Causes of Death, Years, All Races, Both Sexes, U.S., 2003 (Number in population = 39,872,598
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48. Age-Specific Rate (Ri)
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49. Method for Calculation of Age-Specific Death Rates
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50. Adjusted Rates
Summary measures of the rate of morbidity and mortality in a population in which statistical procedures have been applied to remove the effect of differences in composition of various populations.
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51. Direct Method
Direct method may be used if age-specific death rates in a population to be standardized are known and a suitable standard population is available.
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52. New Standard Population
Year 2000 population
Replaces the standard based on 1940 population.
Results in age-adjusted death rates that are much larger.
Affects trends in age-adjusted rates for certain causes of death.
Narrows race differentials in age-adjusted death rates.
Reduces the three different standards into one acceptable standard.
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53. Direct Method for Adjustment of Rates
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54. Weighted Method for Direct Rate Adjustment
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55. Indirect Method
Indirect method may be used if age-specific death rates of the population for standardization are unknown or unstable, for example, because the rates to be standardized are based on a small population.
The standardized mortality ratio (SMR) can be used to evaluate the results of the indirect method.
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56. Standardized Mortality Ratio (SMR)
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57. Interpretation of SMR
If the observed and expected numbers are the same, the SMR would be 1.0, indicating that observed mortality is not unusual.
An SMR of 2.0 means that the death rate in the study population is two times greater than expected.
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58. Illustration of Indirect Age Adjustment: Mortality Rate Calculation for a Fictitious Population of 230,109 Persons
SMR is (502/987.9) X 100 = 50.8%.
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