1. Epidemiology: The
Study of Disease,
Injury, and Death in the
Community
Chapter 3

2. Introduction
• Epidemiology = population medicine
• Epidemiologists concerned with course of
disease in a population
• Collect information about disease status of a
community
• How many people are sick? Who is sick?
When did they become sick? Where do they
live?
• Data can be used to prevent disease outbreaks
or determine effectiveness of prevention effort

3. Definitions
• Epidemiology: study of distribution and
determinants of health-related states or events in
specified populations
• Epidemic: unexpectedly large number of cases of
an illness, specific health-related behavior or
event, in a particular population
• Endemic: disease that occurs regularly in a
population as a matter of course
• Pandemic: outbreak over wide geographic area

4. The Importance of Rates
• Rates allow for comparison of outbreaks at
different times or in different places
• Cases: people afflicted (those who are sick)
• Rates: number of events in a given population
over a given period of time or given point in time
• Natality (birth), morbidity (sickness), mortality or
fatality (death) rates
• Population at Risk: those susceptible to particular
disease or condition

5. Morbidity Rates
• Incidence rate: number of new health-related
events or cases of a disease in a population
exposed to that risk during a particular period of
time, divided by total # in same population
• Prevalence rate: number of new and old cases in
a given period of time, divided by total # in that
population
• Attack rate: incidence rate calculated for a
particular population for a single disease
outbreak; expressed as a percentage

6. Incidence, Prevalence, and Attack Rates
• Incidence rates important to study of acute
diseases
• Acute disease: lasts three months or less
• Prevalence rates useful for study of chronic
diseases
• Chronic disease: last longer than three months
• Incidence and prevalence rates expressed as
crude or specific

7. Incidence, Prevalence, and Attack Rates

8. Crude and Age-Adjusted Rates
• Crude rates: denominator includes the total
population
• Crude birth rate: # of live births in given year,
divided by midyear population
• Crude death rate: # of deaths in given year
from all causes, divided by midyear population
• Age-adjusted rates: used to make comparisons
of relative risks across groups and over time
when groups differ in age structure

9. Crude Rates

10. Crude and Age-Adjusted Rates

11. Specific Rates
• Measure morbidity and mortality for particular
populations or diseases
• Case fatality rate: percentage of cases of a
particular disease that result in death
• Proportionate mortality ratio: percentage of
overall mortality in a population that is
attributable to a particular cause

12. Important Rates in Epidemiology

13. Reporting of Birth, Deaths, and Diseases
• Physicians, clinics, and hospitals required to
report births, deaths, and notifiable diseases
• Notifiable diseases: infectious diseases in
which health officials request or require
reporting
• Can become epidemics
• Health officials maintain weekly records
• Various challenges to maintaining accurate
data

14. Notifiable Disease Scheme

15. Standardized Measurements of Health Status of
Populations
• Mortality statistics most reliable measure of
population health status
• Easier to track death than illness
• Life expectancy: average number of years a
person from a specific cohort is projected to
live from a given point in time
• Years of potential life lost (YPLL): number of
years lost when death occurs before one’s life
expectancy

16. Life Expectancy Figures

17. Years of Potential Life Lost
• Subtract person’s age at death from his or her
life expectancy
• Difficult to determine because life expectancy
changes at different ages
• Weighs death of young person as counting
more than death of old

18. Sources of Standardized Data
• Various valid sources have specific value and
usefulness to those in public health
• U.S. Census
• Taken every 10 years
• Enumeration of the population
• More complex now
• Gathers data on race, age, income,
employment, education, and other social
indicators

19. Sources of Standardized Data (ctd)
• Statistical Abstract of the United States
• Book published annually by Bureau of Census
• Summary of statistics on social, political, and
economic organization of the United States
• Monthly Vital Statistics Report
• Vital statistics are summaries of records of
major life events: birth, death, marriage, divorce
• Published by National Center for Health
Statistics under the CDC
• Also calculates death rates by race and age

20. Sources of Standardized Data (ctd)
• Morbidity and Mortality Weekly Report
(MMWR)
• Prepared by CDC from state health department
reports
• Reports morbidity and mortality data by state
and region of U.S.
• Reports outbreaks of disease, environmental
hazards, unusual cases, or other public health
problems

21. National Health Surveys
• National Health Survey Act of 1956
authorized continuing survey of amount,
distribution, and effects of illness and
disability in the U.S.
• Three types of surveys
• Health interviews of people
• Clinical tests, measurements, and physical
examinations
• Surveys of places where people receive
medical care

22. Some National Health Surveys
• National Health Interview Survey (NHIS)
• Conducted by NCHS
• Questions respondents about their health
• National Health and Nutrition Examination
Survey (NHANES)
• Assesses health and nutrition status through
mobile examination center
• Behavioral Risk Factor Surveillance System
(BRFSS); National Health Care Survey (NHCS)

23. Epidemiological Studies
• Investigations carried out when disease or
death occurs in unexpected or unacceptable
numbers
• Descriptive studies
• Describe epidemics with respect to person,
place, and time
• Analytic studies
• Aimed at testing hypotheses

24. Descriptive Studies
• Who?
• Case count, followed by who is ill (children,
men, women, race, etc.)
• When?
• Time of onset for each case
• Epidemic curves created
• Where?
• Determine residential address and travel history

25. Epidemic Curves
• Graphic display of the cases of disease
according to the time or date of onset of
symptoms
• Secular, seasonal, and single epidemic curves
• Single epidemic curves
• Point source epidemic curve
• Propagated epidemic curve

26. Secular Epidemic Curve

27. Seasonal Epidemic Curve

28. Point Source Epidemic Curve

29. Propagated Epidemic Curve

30. Analytic Studies
• Test hypotheses about relationships between
health problems and possible risk factors
• Observational studies: investigator observes
natural course of events, noting exposed vs.
unexposed and disease development
• Case/control studies
• Cohort studies
• Experimental studies: investigator allocates
exposure and follows development of disease

31. Case/Control Studies
• Case/Control
• Compares those with disease to those without
but with similar background and/or with prior
exposure to certain risk factors
• Aimed at identifying factors more common in
case than control group

32. Cohort Studies
• Cohort is classified by exposure to one or
more risk factors and observed to determine
rate of disease development
• Cohort: group of people who share important
demographic characteristic
• Odds ratio
• Relative risk

33. Experimental Studies
• Carried out to identify cause of disease or
determine effectiveness of vaccine, drug, or
procedure
• Control for variables
• Control groups
• Randomization
• Blinding
• Placebo: blank treatment

34. Criteria of Causation
• Questions exposure causing development of
disease
• Criteria
• Strength
• Consistency
• Specificity
• Temporality
• Biological plausibility

35. Discussion Questions
• How can data collection for notifiable diseases
be improved?
• Why is tracking vital statistics so important?
• How does calculating Years of Potential Life
Lost change the way we think about
community health efforts?