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 A.Dey@fhs.usyd.edu.auA.Dey@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