Unit 2 – Public Health
Epidemiology
Chapter 4 – Epidemiology: The Basic Science of
Public Health
Epidemiology
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The diagnostic discipline of public health
A major part of public health’s assessment function
Investigates causes of diseases
Identifies trends in disease occurrence
Evaluates effectiveness of medical and public health
interventions
 An observational science
Patterns of Disease Occurrence
 Who is getting the disease?
 When did they get the disease?
 Where is the disease occurring?
 From this information, epidemiologists can infer why
the disease is occurring.
Epidemic Surveillance
 Endemic vs. Epidemic
 Notifiable diseases
 Recognition of new disease
 Increased importance with threat of bioterrorism
Outbreak Investigation
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Verify the diagnosis
Construct a working case definition
Find cases systematically – active surveillance
Ask the who, where, and when questions to describe
the epidemic by person, place, and time. Consider the
incubation period
 Look for common source of exposure
Question #1
 What has epidemiology contributed to peoples
understanding of
 Heart Disease?
 Lung Cancer?
Epidemiology and Chronic Diseases
 Identify risk factors
 Observe long-term trends
 Heart disease – leading cause of death in U.S.
 Framingham Study – started in 1948
 Lung cancer and smoking – early 1950s
 British Physicians’ Study
 Hammond-Horn study in U.S.
Unit 2 – Public Health
Epidemiology
Chapter 5 – Epidemiologic Principles and Methods
Definition of Epidemiology
 Epidemiology is the study of the distribution and
determinants of disease frequency in human
populations
Step 1: Define the Disease
 Death is easy to determine – death certificates
have cause of death
 Some diseases need blood tests or stool cultures to
verify diagnosis
 Some diseases are hard to define – e.g. EMS, SARS
 Sometimes definition changes as more is learned –
e.g. AIDS
 Other health outcomes – injuries, risk factors
Question #2
 Explain the interaction between incidence, prevalence
and prognosis. Give examples
Disease Frequency
 Count number of people with disease and relate to
the population at risk (PAR)
 PAR (denominator) may be total population or
exposed population, or one gender or age group;
often comes from census
 Two ways to measure frequency:
 Incidence – number of new cases
 Prevalence – number of existing cases
 Incidence is used for studying causes of disease
Disease Frequency, ctd.
 If causes or risk factors increase, incidence and
prevalence increase
 If ability to diagnose increases, incidence and
prevalence increase
 Prevalence depends on incidence and prognosis
Question #3
 Why are the who, when, and where questions useful in
determining the causes of disease? Give examples.
Distribution of Disease
 Who – sex, age, occupation, race
 When – season, year (long-term trends), elapsed time
since an exposure (epidemic curve)
 Where – neighborhood (e.g. clusters), latitude
(climate), urban vs. rural, national variations
Determinants of Disease
 Why is distribution as it is?
 Can make inferences from distribution
 Risk factors
Human Population
 Epidemiology observes humans, in contrast to
biomedical sciences, which can do experiments on
laboratory animals
Question #4
 Explain the three major types of epidemiologic
studies.
Kinds of Epidemiologic Studies
 Goal is to determine an association between an
exposure and a disease or other health outcome
 May be prospective or retrospective
 Intervention study
 Cohort study
 Case-control study
Question #5
 Which is most likely to yield a valid result? Why?
Intervention Study
 Closest thing to an experiment
 Start with two groups: experimental group (gets
the intervention or exposure) and control group
 Watch them over time and compare outcomes
 Experimenter chooses who is in which group
 Two groups should be as similar as possible so that
intervention is the only difference
Intervention Study, ctd.
 Randomized, double-blind, placebo control is the
ideal
 Pharmaceutical companies conduct many clinical
trials for new drugs
 Physicians’ Health Study – prevention study
 Aspirin to prevent heart disease
 Beta carotene to prevent cancer
 Field trial of polio vaccine – 1954
 Kingston-Newburgh study of fluoridation to
prevent tooth decay
Cohort Study
 For situations when doing an intervention study would
be unethical or too difficult
 Considered the next most accurate
 Choose a large number of healthy people, collect data
on their exposures, and track outcomes over time
 The only difference from intervention is that people
choose their own exposures
Cohort Study -- examples
 Framingham Heart Study
 Nurses’ Health Study
 British physicians study on smoking and lung cancer
 Hammond-Horn study on smoking and lung cancer in
U.S.
Question #6
 Which is likely to yield an answer in the shortest
period of time? Why?
Case-Control Study
 Choose people who already have disease
 Choose a healthy control group of individuals as
similar as possible to cases
 Interview them all and ask for their previous exposures
 Advantage: faster and cheaper
 Least accurate approach
Unit 2 – Public Health
Epidemiology
Chapter 6 – Problems and Limits of Epidemiology
Problems with Studying Humans
 Intervention study: subjects may not follow
prescribed behavior throughout study period
 Cohort study: sometimes hard to isolate which of
many factors are responsible for health differences
 Case-control study: control group may not be
truly comparable; also errors in reporting or recall
 For all studies, must worry about differential dropouts
Sources of Error
 Random variation
 Confounding variables
 Bias
 Selection bias
 Reporting bias or recall bias
Factors that Lend Validity to
Results
 Strong association
 Dose-response relationship
 Known biological explanation
 Large study population
 Consistent results from several studies
Hormone Replacement Therapy
 Conflicting results from two major studies
 Clinical trial is the gold standard
 Results of cohort study were confounded by associated
factors that made women taking HRT healthier, even
without the therapy
Ethical Issues
 Nazi experiments on humans
 Tuskegee syphilis study
 New rules – informed consent
 New rules -- institutional review boards
 Importance of clinical trials
 Possibility of conflict of interest by medical providers
who stand to profit
Question #7
 Look up the Tuskegee Syphilis Study? Why was it
unethical? What influence has it had on the conduct
of clinical trials?