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Epidemiology
Learning Objectives for the Learning Objectives for the UnitUnit
Having completed this unit, the student should be Having completed this unit, the student should be able to:able to:
• Define epidemiology and list its usesDefine epidemiology and list its uses• Describe the key features of epidemiologyDescribe the key features of epidemiology• Describe the basic measures used in Describe the basic measures used in
epidemiologyepidemiology• Calculate measures of disease frequency Calculate measures of disease frequency • Interpret epidemiological data Interpret epidemiological data • Define and calculate measures of associationDefine and calculate measures of association
Learning Objectives for the Learning Objectives for the UnitUnit• Describe study types used in epidemiologyDescribe study types used in epidemiology• Identify the major sources of bias in research studiesIdentify the major sources of bias in research studies• Define the keys elements of a health surveillance Define the keys elements of a health surveillance
system system • Describe the key aspects of an outbreak investigationDescribe the key aspects of an outbreak investigation• Assess information on the effectiveness of screening Assess information on the effectiveness of screening
programsprograms• Assess the overall quality of a published report of an Assess the overall quality of a published report of an
epidemiological studyepidemiological study
Unit – 4 credit pointsUnit – 4 credit points• Average student input per week:
5-6 hours • Lectures (2 hours each week) • Tutorials (1 hour each week).
• It is expected that students will attend classes.
AssessmentAssessment
• Sectional Test Sectional Test Week 9Week 9 25% 25%
• GroupGroupAssignment Assignment Week 12Week 12 25% 25%
• Final Examination Week 15/16 Final Examination Week 15/16 50%50% (2 hours)(2 hours)
All assessments must be completed and All assessments must be completed and submitted to pass the unit.submitted to pass the unit.
TextText• Gordis, L. Gordis, L. Epidemiology.Epidemiology. WB WB
Saunders Co., Philadelphia, 2004 Saunders Co., Philadelphia, 2004 (3(3rdrd Edition) Edition)
Other ReadingsOther Readings
Contact detailsContact details
Dr Aditi DeyDr Aditi Dey
Room T337, Level 3, T BlockRoom T337, Level 3, T BlockPhone 9351 9058, 9351 9494Phone 9351 9058, 9351 9494Email Email [email protected]@fhs.usyd.edu.au
Lecture OutlineLecture Outline
• IntroductionIntroduction• Population Health & Disease Measures Population Health & Disease Measures • Causation and Epidemiological Study Types Causation and Epidemiological Study Types • Cohorts and Case- Control Studies and Cohorts and Case- Control Studies and
measures of associationmeasures of association• Randomised Controlled TrialsRandomised Controlled Trials• Ecological studies and Cross Sectional Ecological studies and Cross Sectional
SurveysSurveys
Lecture OutlineLecture Outline• Sources of Error: Sources of Error:
– Selection & MeasurementSelection & Measurement
• ConfoundingConfounding• Health Surveillance Health Surveillance • Disease OutbreaksDisease Outbreaks• Critical AppraisalCritical Appraisal• ScreeningScreening• Using Epidemiology to Evaluate Health Using Epidemiology to Evaluate Health
Services/ReviewServices/Review
Learning Objectives for Learning Objectives for todaytoday
• Define epidemiologyDefine epidemiology
• List the uses of epidemiologyList the uses of epidemiology
• Describe key historical epidemiological Describe key historical epidemiological studiesstudies
• Explain the key features of Explain the key features of epidemiological studiesepidemiological studies
Origin of the term Origin of the term ‘epidemiology’‘epidemiology’
• epi - ‘on, upon, at, by, near, over, on top epi - ‘on, upon, at, by, near, over, on top of, against, among’of, against, among’
• demos - ‘common people or citizenry’demos - ‘common people or citizenry’
• ology - ‘the study of’ology - ‘the study of’
• epidemiology =‘Study of disease among epidemiology =‘Study of disease among the population’the population’
Definition of EpidemiologyDefinition of Epidemiology
• ‘‘study of the study of the distributiondistribution and and determinantsdeterminants of health related states or events in of health related states or events in specified populationsspecified populations and the application and the application of this study to the control of health of this study to the control of health problems’ problems’
Last, 1995Last, 1995
Epidemiology is about Epidemiology is about PopulationsPopulations
• Groups of people not individualsGroups of people not individuals
• It answers population questionsIt answers population questions– aetiology of disease aetiology of disease – prevention of diseaseprevention of disease– Extent/distribution of disease (allocation Extent/distribution of disease (allocation
of effort & resources in health facilities of effort & resources in health facilities and communities)and communities)
Epidemiology and Clinical Epidemiology and Clinical PracticePractice
Practice of medicine is dependent on Practice of medicine is dependent on population data. For example:population data. For example:
• A physician hears a apical systolic murmur A physician hears a apical systolic murmur and knows it represents mitral regurgitation.and knows it represents mitral regurgitation.
• How does this knowledge originate?How does this knowledge originate?• The The diagnosisdiagnosis is based on association of is based on association of
auscultatory findings with the findings of auscultatory findings with the findings of surgical pathology or autopsy in a surgical pathology or autopsy in a large large group of patients. group of patients. Hence, diagnosis is Hence, diagnosis is population basedpopulation based
Example: PrognosisExample: PrognosisA patient may ask a doctor, “How long will I A patient may ask a doctor, “How long will I
live?’’live?’’The doctor usually answers on the basis of :The doctor usually answers on the basis of :• ExperienceExperience with large with large groups of patientsgroups of patients
who had the same diseasewho had the same disease• Were observed at the same stage of Were observed at the same stage of
disease and received the same treatmentdisease and received the same treatment
Epidemiology and Clinical Medicine
Relationship between
Studies/Assessments
Prevention
Evaluation
Planning
Diagnosis
Treatment
Cure
Care
Examples of Epidemiological StudiesExamples of Epidemiological Studies(Use of observational data for (Use of observational data for prevention of disease)prevention of disease)
VaccinationVaccination: Prevention of smallpox: Prevention of smallpox
• Edward Jenner observed that dairy maids Edward Jenner observed that dairy maids (women who milked cows) developed a mild (women who milked cows) developed a mild disease called cowpox.disease called cowpox.
• Later, in outbreaks of smallpox, these dairy Later, in outbreaks of smallpox, these dairy maids did not develop smallpox and overheard maids did not develop smallpox and overheard one of them say, “I can’t take smallpox for I one of them say, “I can’t take smallpox for I already had the cowpox.’’ This information was already had the cowpox.’’ This information was observational.observational.
• Jenner decided to test the hypothesis that Jenner decided to test the hypothesis that cowpox could provide protection against cowpox could provide protection against smallpox.smallpox.
Examples of Epidemiological Examples of Epidemiological StudiesStudies
• Vaccination – Vaccination – Edward Jenner Edward Jenner & the smallpox & the smallpox vaccinationvaccination
First vaccinationFirst vaccination•Jenner took cowpox material from a dairy maid (Sarah Nelmes) and administered to an 8-year old James Phipps
•6 weeks later, Jenner inoculated the child with material now taken from a smallpox pustule.
•The child did not develop smallpox
• Jenner knew nothing about the Jenner knew nothing about the biology of the disease or about biology of the disease or about virusesviruses
• He based his hypothesis from purely He based his hypothesis from purely observational data observational data
Examples of Epidemiological Examples of Epidemiological StudiesStudies• 1854 Outbreak of Cholera in London: 1854 Outbreak of Cholera in London:
sign at cemetery in Dudley, Englandsign at cemetery in Dudley, England
Epidemiology of choleraEpidemiology of cholera
• John Snow and epidemiology of John Snow and epidemiology of choleracholera
• John Snow: anaesthesiologist who John Snow: anaesthesiologist who administered chloroform to Queen administered chloroform to Queen Victoria in childbirthVictoria in childbirth
• In the 1In the 1stst week of September, 1854 week of September, 1854 about 600 people died from cholera. about 600 people died from cholera.
• These people lived within a few blocks These people lived within a few blocks of the Broad Street pump in Londonof the Broad Street pump in London
Cholera outbreak - mapCholera outbreak - map
• John Snow believed that cholera was caused by John Snow believed that cholera was caused by contaminated watercontaminated water (others had other theories: (others had other theories: miasmatic theory)miasmatic theory)
• In London at that time, a person obtained water In London at that time, a person obtained water by signing up with one of the water supply by signing up with one of the water supply companies.companies.
• The intake for the water companies was a very The intake for the water companies was a very polluted part of Thames river.polluted part of Thames river.
• One of the companies (One of the companies (Lambert CompanyLambert Company) ) shifted its water intake upstream in the Thames shifted its water intake upstream in the Thames to a less polluted part of the river but the others to a less polluted part of the river but the others didn’t.didn’t.
• Snow hypothesised that: Snow hypothesised that: mortalitymortality from from choleracholera would bewould be lower lower in people getting water from in people getting water from Lambert CompanyLambert Company than those getting water than those getting water from other companiesfrom other companies
Rates of cholera deathsRates of cholera deaths
Water SupplyWater Supply Cholera Deaths per Cholera Deaths per 10,000 houses10,000 houses
Southwalk & Southwalk & VauxhallVauxhall
315315
Lambeth CoLambeth Co 3838
Other districts in Other districts in LondonLondon
5656
TheThe Broad Street pump Broad Street pump After the panic-stricken officials followed Snow's advice to remove the handle of the Broad Street Pump that supplied the water to this neighbourhood, the epidemic was contained.
Examples of Epidemiological Examples of Epidemiological StudiesStudies
• Link between smoking and lung cancerLink between smoking and lung cancer
Doll & Hill, 1964
Examples of Epidemiological Examples of Epidemiological StudiesStudiesWater fluoridation:
•Communities that had low natural water fluoride levels had high levels of dental caries
•Communities that had high natural water fluoride levels had low levels of dental caries
Trial in New York communities: Trial in New York communities: Newburg and KingstonNewburg and Kingston
• DMF (decayed, missed and filled) DMF (decayed, missed and filled) index was used.index was used.
• Baseline information from both Baseline information from both communities collectedcommunities collected
• Water in Newburg was fluoridated Water in Newburg was fluoridated and children re-examinedand children re-examined
• DMF index in Newburg had dropped DMF index in Newburg had dropped after 10 yearsafter 10 years
Rate of decayed teeth – Rate of decayed teeth – regional differencesregional differences
• Water fluoridation is a controversial Water fluoridation is a controversial issue so after fluoride was added to its issue so after fluoride was added to its water supply, it was discontinued after water supply, it was discontinued after a referendum in Wisconsin (USA)a referendum in Wisconsin (USA)
• The next figure shows that after The next figure shows that after fluoride was removed, the DMF index fluoride was removed, the DMF index roserose
• This provided further evidence that This provided further evidence that fluoride acted to prevent dental caries fluoride acted to prevent dental caries
Decayed teeth – after Decayed teeth – after fluoridisationfluoridisation
Epidemiological QuestionsEpidemiological Questions
• When can we expect the next flu epidemic?When can we expect the next flu epidemic?• Are the number of AIDS cases increasing Are the number of AIDS cases increasing
or decreasing?or decreasing?• Should we screen the male population for Should we screen the male population for
prostate cancer?prostate cancer?• How can cervical cancer best be How can cervical cancer best be
prevented?prevented?• Has the slip, slap, slop campaign reduced Has the slip, slap, slop campaign reduced
skin cancers rates?skin cancers rates?
Uses of Uses of Epidemiology(Gordis, 2000)Epidemiology(Gordis, 2000)• Identifies aetiology or causes of disease including the Identifies aetiology or causes of disease including the
risk factors for the disease.risk factors for the disease.
• Determine the extent of the disease in the communityDetermine the extent of the disease in the community
• Examines natural history of disease and prognosis of Examines natural history of disease and prognosis of disease disease
• Investigates and controls disease outbreaksInvestigates and controls disease outbreaks
Uses of Uses of Epidemiology(Gordis, 2000)Epidemiology(Gordis, 2000)
• Describes and monitors the population health and the Describes and monitors the population health and the patterns of disease patterns of disease
• Evaluates new preventive and therapeutic Evaluates new preventive and therapeutic interventions and modes of health care deliveryinterventions and modes of health care delivery
• Provides information to inform public policy decisionsProvides information to inform public policy decisions
Identifies aetiology or causes of Identifies aetiology or causes of disease including the risk factors disease including the risk factors for the disease.for the disease.
Doll and Hill,1964Doll and Hill,1964
Determine the extent of the Determine the extent of the disease in the communitydisease in the community
NSW Health Survey,1997NSW Health Survey,1997
Examines natural history of Examines natural history of disease and prognosis of disease and prognosis of diseasedisease
Population at
Risk
Development of
End-Stage
Complications
(Blindness, ESRF, Heart Attack, Amputations)
Primary
Prevention
Increase Activity
Improve Nutrition
Weight Control
Modifiable:
Poor glucose control
Inactivity
Obesity
Fat Intake
Hypertension
Dyslipidaemia
Poor Footcare
Smoking
Non-Modifiable:
Genetics
Disease Duration
Modifiable:
Undiagnosed onset of complications
Untreated progression of complications
Poor glucose control
Hypertension
Dyslipidaemia
Poor Footcare
Smoking
Non-Modifiable:
Genetics
Risk Factors
Modifiable:
Inactivity
Obesity
Non-Modifiable:
Genetics
Age
Natural Historyof NIDDM
Possible Intervention Points
Screen for NIDDM
in at risk populations
Manage NIDDM
Glucose Control
Education
Increase Activity
Reduce ObesityDiet Modification
Treat Hypertension
Treat Hyperlipidaemia
Self care
Reduce Smoking
Treat Complications
Glucose Control
Education
Increase Activity
Reduce Obesity
Treat Hypertension
Treat Hyperlipidaemia
Treat RetinopathyTreat Vascular Problems
Self care
Reduce Smoking
Figure 1: The Natural History of NIDDM
Ongoing
Screening for
complications
EyesFeet
CVD
Kidneys
Healthy
PopulationOnset of complications
Onset of NIDDM
(Often asymptomatic)
Rehabilitation and
PalliationDialysis
Post Amputation
Rehab
Cardiac Rehab
Stroke Rehab
Aids
Support
Investigates and controls Investigates and controls disease outbreaksdisease outbreaks
Describes and monitors the Describes and monitors the population health and the patterns population health and the patterns
of diseaseof disease
NSW Cancer Registry, 2002
Describes and monitors the Describes and monitors the population health and the patterns population health and the patterns
of diseaseof disease – breast cancer– breast cancer
NSW Cancer Registry, 2002
Evaluates new preventive and Evaluates new preventive and therapeutic interventions and therapeutic interventions and modes of health care deliverymodes of health care delivery
•Has growth of managed care and other new approaches to health care delivery had an impact on the patients’ QOL?
•Does screening women with MRI for breast cancer improve survival for women?
Provides information to inform Provides information to inform public policy decisions - public policy decisions - Smoking in restaurantsSmoking in restaurantsSmokefree Environment BillSmokefree Environment Bill, , 20002000
NSW Health Survey,1997
Determining CausationDetermining Causation
• Disease has been classically Disease has been classically described as a result of an described as a result of an epidemiological triadepidemiological triad
• It is the It is the product product of an of an interaction interaction of of the human host, an agent (eg the human host, an agent (eg infectious) and the environment that infectious) and the environment that promotes the exposurepromotes the exposure
• For such an interaction to occur, the For such an interaction to occur, the host must be susceptiblehost must be susceptible
Determining CausationDetermining Causation
• Environment FactorsEnvironment Factors physical, social or biological environment circumstancesphysical, social or biological environment circumstances
• Host FactorsHost Factors – susceptibility eg immunity, behaviours – susceptibility eg immunity, behaviours • Vectors of diseasesVectors of diseases: insects such as mosquitoes, : insects such as mosquitoes,
arthropods (ticks) etcarthropods (ticks) etc
Disease
Agent
Host Environment
• Agent(s)eg microorganisms, chemicals, psychological factors
Determining Causation - Determining Causation - InfectionInfection
Disease: Disease: Acute rheumatic feverAcute rheumatic fever (polyarthritis, subcutaneous nodules over bony (polyarthritis, subcutaneous nodules over bony
prominences, myocarditis which may cause acute prominences, myocarditis which may cause acute cardiac failure & endocarditis)cardiac failure & endocarditis)
Agent: Beta-haemolyticstreptococcus
Host:Low immunity
EnvironmentPoverty, climate
Disease
Determining Causation - Determining Causation - Chronic DiseaseChronic Disease
- can be more complexcan be more complex
- usually the result of many factors (referred usually the result of many factors (referred to as risk factors) acting in sequence, or to as risk factors) acting in sequence, or together that result in disease in an together that result in disease in an individual. individual.
Causal Pathway or Causal Causal Pathway or Causal WebWeb
Source: Stallones, R.A. (1966)
Risk Factor is:Risk Factor is:
• An aspect of An aspect of – personal behaviour or personal behaviour or – Lifestyle orLifestyle or– environmental exposure or environmental exposure or – an inborn or inherited characteristic an inborn or inherited characteristic
that is that is associatedassociated with an with an increased increased occurrence of diseaseoccurrence of disease or other health- or other health-related event or condition. related event or condition.
Risk Factor (major categories)Risk Factor (major categories)BEINGSBEINGS model by Jekel et al 2001 model by Jekel et al 2001
• BBiological & behavioural factorsiological & behavioural factors
• EEnvironmental factorsnvironmental factors
• IImmunological factorsmmunological factors
• NNutritional factorsutritional factors
• GGenetic factorsenetic factors
• SServices, Social & Spiritual factorservices, Social & Spiritual factors
Defining Populations in Defining Populations in EpidemiologyEpidemiology
• Epidemiology focuses on Epidemiology focuses on defineddefined population and measures disease outcomes population and measures disease outcomes & health in relation to a & health in relation to a population at riskpopulation at risk
• Population at Risk = People, healthy or sick Population at Risk = People, healthy or sick have potential to develop a particular health have potential to develop a particular health state or illness.state or illness.
• They would be counted as They would be counted as casescases if they had if they had the disease being studiedthe disease being studied
ExampleExample• In a survey of hang-gliding accidents it was recommended In a survey of hang-gliding accidents it was recommended
that flying should be banned between 11am and 3pm that flying should be banned between 11am and 3pm because this was when the greatest number (86%) of because this was when the greatest number (86%) of accidents occurredaccidents occurred
• During the month of May 1995:During the month of May 1995:– 200 hang-gliders flew between the hours of 11am and 200 hang-gliders flew between the hours of 11am and
3pm3pm– 50 accidents occurred during this time50 accidents occurred during this time– 10 people flew outside the hours of 11am & 3pm10 people flew outside the hours of 11am & 3pm– 8 accidents occurred during this time8 accidents occurred during this time
• Given this information, determine what time you are most Given this information, determine what time you are most likely to have an accident?likely to have an accident?
ExampleExample
• Accidents during 11am and 3pmAccidents during 11am and 3pm– 50 accidents per 200 people gliding 50 accidents per 200 people gliding – 1 in 4 people = 25%1 in 4 people = 25%
• Accidents outside the hours of 11am & Accidents outside the hours of 11am & 3pm3pm– 8 accidents 10 people flew occurred during 8 accidents 10 people flew occurred during
this timethis time– 4 in 5 people = 80%4 in 5 people = 80%
Types of Populations Types of Populations
• Population at RiskPopulation at Risk = People, = People, healthy or sick who have the healthy or sick who have the potential to develop a potential to develop a particular health state or particular health state or illness.illness.
• Populations may also be Populations may also be defined as: defined as: – cohort or cohort or
– dynamic populationsdynamic populations
Cohort Populations Cohort Populations
• Also called Also called fixed populationsfixed populations
• Criteria for membership: experienced Criteria for membership: experienced an event or set of eventsan event or set of events
• Once the qualifying events have been Once the qualifying events have been experienced by the person, membership experienced by the person, membership of the cohort becomes a permanentof the cohort becomes a permanent
• Examples: Examples: – a birth cohort may consist of all people a birth cohort may consist of all people
born in 1973; born in 1973; – people who have a tertiary degree.people who have a tertiary degree.
Dynamic PopulationDynamic Population
• Have to meet certain criteria to be Have to meet certain criteria to be part of the populationpart of the population
• Membership only lasts while the Membership only lasts while the qualifying criteria are present, qualifying criteria are present, membership may be only membership may be only temporarytemporary
• Examples: Examples: – people attending university – people attending university –
membership is temporary membership is temporary
Dynamic PopulationDynamic Population
• Can be in a Can be in a steady statesteady state when a when a specific feature remains constant specific feature remains constant despite changing membership.despite changing membership.
• Example: Example: – if 51% of a dynamic population is female if 51% of a dynamic population is female
and this remains constant over time (eg and this remains constant over time (eg some women die and some women are some women die and some women are born) - the population has a steady state in born) - the population has a steady state in respect to gender compositionrespect to gender composition
• Are people in clinical trials a Are people in clinical trials a cohort or dynamic population?cohort or dynamic population?
Key components of Key components of epidemiological studiesepidemiological studies
StudyPopulation/
Sample
Exposure to a study factor
Outcome
Unexposed
Exposed
Target Population
Key components of Key components of epidemiological studiesepidemiological studies• Target populationTarget population is the population a is the population a
researcher wants to make researcher wants to make generalizations about generalizations about
• Study populationStudy population is the group a is the group a researcher wishes to study (sometimes researcher wishes to study (sometimes the same as the target population)the same as the target population)
• Study sampleStudy sample is a group of subjects is a group of subjects chosen for study to represent the study chosen for study to represent the study populationpopulation
Key components of Key components of epidemiological studiesepidemiological studies
StudyPopulation/
Sample
Exposure to a study factor
Outcome
Unexposed
Exposed
Target Population
Key components of Key components of epidemiological studiesepidemiological studies• Study factorStudy factor
– is a element that is being investigated to see if it is is a element that is being investigated to see if it is a a determinantdeterminant of a particular health problem of a particular health problem
– or if it reduces the impact of a particular health or if it reduces the impact of a particular health problem. problem.
– Study factors can include Study factors can include • risk factors for a health problem,risk factors for a health problem,• interventions (therapeutic or interventions (therapeutic or
preventative) to ameliorate a health preventative) to ameliorate a health condition, condition,
• diagnostic tests or techniques and diagnostic tests or techniques and • environmental exposures.environmental exposures.
• ExposureExposure is contact with or possessing a is contact with or possessing a particular study factorparticular study factor
• Exposed group Exposed group is a is a groupgroup whose whose members have had contact with or members have had contact with or possess a study factorpossess a study factor
Key components of Key components of epidemiological studiesepidemiological studies• UnexposedUnexposed groupgroup is a group that has is a group that has
not had contact with a cause of, or not had contact with a cause of, or possess a characteristic that is a possess a characteristic that is a determinant of, a particular health determinant of, a particular health problem.problem.
• OutcomeOutcome is any or all of the possible is any or all of the possible results that may stem from an exposure results that may stem from an exposure or study factor.or study factor.
Conclusions Based on Conclusions Based on ComparisonsComparisons
• Clues to aetiology come from comparing Clues to aetiology come from comparing disease rates in groups with differing disease rates in groups with differing levels of exposurelevels of exposure
• Clues will be missed or false clues created Clues will be missed or false clues created if comparisons are biased by unequal if comparisons are biased by unequal collection of cases or exposure levelscollection of cases or exposure levels
Proportion of Deaths Attributable to Proportion of Deaths Attributable to Tobacco, NSW, 1996-1998 (Close & Tobacco, NSW, 1996-1998 (Close & Achat)Achat)
Cause of DeathCause of Death Western Western SydneySydney
NSWNSW
Lung CancerLung Cancer 31.331.3 29.329.3
Other cancerOther cancer 8.58.5 9.09.0
Coronary heart Coronary heart diseasedisease
22.922.9 20.620.6
RespiratoryRespiratory
diseasedisease
24.924.9 27.727.7
Next weekNext week
• Population Health and Disease Population Health and Disease MeasuresMeasures– incidenceincidence– prevalenceprevalence– death ratesdeath rates– standardisationstandardisation
Text: Text: P31-59P31-59