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Unit 2 – Public HealthEpidemiology
Chapter 4 – Epidemiology: The Basic Science of Public Health
EpidemiologyThe diagnostic discipline of public healthA major part of public health’s assessment
functionInvestigates causes of diseasesIdentifies trends in disease occurrenceEvaluates effectiveness of medical and public
health interventionsAn observational science
Patterns of Disease OccurrenceWho 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 SurveillanceEndemic vs. EpidemicNotifiable diseasesRecognition of new diseaseIncreased importance with threat of
bioterrorism
Outbreak InvestigationVerify the diagnosisConstruct a working case definitionFind cases systematically – active
surveillanceAsk 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 #1What has epidemiology contributed to
peoples understanding of Heart Disease?Lung Cancer?
Epidemiology and Chronic DiseasesIdentify risk factorsObserve long-term trendsHeart disease – leading cause of death in U.S.
Framingham Study – started in 1948Lung cancer and smoking – early 1950s
British Physicians’ StudyHammond-Horn study in U.S.
Unit 2 – Public HealthEpidemiology
Chapter 5 – Epidemiologic Principles and Methods
Definition of EpidemiologyEpidemiology is the study of the distribution
and determinants of disease frequency in human populations
Step 1: Define the DiseaseDeath is easy to determine – death
certificates have cause of deathSome diseases need blood tests or stool
cultures to verify diagnosisSome diseases are hard to define – e.g.
EMS, SARSSometimes definition changes as more is
learned – e.g. AIDSOther health outcomes – injuries, risk
factors
Question #2 Explain the interaction between incidence,
prevalence and prognosis. Give examples
Disease FrequencyCount 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 casesPrevalence – number of existing cases
Incidence is used for studying causes of disease
Disease Frequency, ctd.If causes or risk factors increase, incidence
and prevalence increaseIf ability to diagnose increases, incidence and
prevalence increasePrevalence depends on incidence and
prognosis
Question #3Why are the who, when, and where questions
useful in determining the causes of disease? Give examples.
Distribution of DiseaseWho – sex, age, occupation, raceWhen – 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 DiseaseWhy is distribution as it is?Can make inferences from distributionRisk factors
Human PopulationEpidemiology observes humans, in contrast
to biomedical sciences, which can do experiments on laboratory animals
Question #4Explain the three major types of
epidemiologic studies.
Kinds of Epidemiologic StudiesGoal is to determine an association between
an exposure and a disease or other health outcome
May be prospective or retrospectiveIntervention studyCohort studyCase-control study
Question #5Which is most likely to yield a valid result?
Why?
Intervention StudyClosest thing to an experimentStart with two groups: experimental group
(gets the intervention or exposure) and control group
Watch them over time and compare outcomesExperimenter chooses who is in which groupTwo groups should be as similar as possible
so that intervention is the only difference
Intervention Study, ctd.Randomized, double-blind, placebo control
is the idealPharmaceutical companies conduct many
clinical trials for new drugsPhysicians’ Health Study – prevention study
Aspirin to prevent heart diseaseBeta carotene to prevent cancer
Field trial of polio vaccine – 1954Kingston-Newburgh study of fluoridation to
prevent tooth decay
Cohort StudyFor situations when doing an intervention
study would be unethical or too difficultConsidered the next most accurateChoose 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 -- examplesFramingham Heart StudyNurses’ Health StudyBritish physicians study on smoking and lung
cancerHammond-Horn study on smoking and lung
cancer in U.S.
Question #6Which is likely to yield an answer in the
shortest period of time? Why?
Case-Control StudyChoose people who already have diseaseChoose a healthy control group of individuals
as similar as possible to casesInterview them all and ask for their previous
exposuresAdvantage: faster and cheaperLeast accurate approach
Unit 2 – Public HealthEpidemiology
Chapter 6 – Problems and Limits of Epidemiology
Problems with Studying HumansIntervention study: subjects may not follow
prescribed behavior throughout study periodCohort 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 drop-outs
Sources of ErrorRandom variationConfounding variablesBias
Selection biasReporting bias or recall bias
Factors that Lend Validity to ResultsStrong associationDose-response relationshipKnown biological explanationLarge study populationConsistent results from several studies
Hormone Replacement TherapyConflicting results from two major studiesClinical trial is the gold standardResults of cohort study were confounded by
associated factors that made women taking HRT healthier, even without the therapy
Ethical IssuesNazi experiments on humansTuskegee syphilis studyNew rules – informed consentNew rules -- institutional review boardsImportance of clinical trialsPossibility of conflict of interest by medical
providers who stand to profit
Question #7Look up the Tuskegee Syphilis Study? Why
was it unethical? What influence has it had on the conduct of clinical trials?
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