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1. Epidemiology

2. Learning Objectives for the Unit
Having completed this unit, the student should
be able to:
• Define epidemiology and list its uses
• Describe the key features of epidemiology
• Describe the basic measures used in
epidemiology
• Calculate measures of disease frequency
• Interpret epidemiological data
• Define and calculate measures of
association

3. Learning Objectives for the Unit
• Describe study types used in epidemiology
• Identify the major sources of bias in
research studies
• Define the keys elements of a health
surveillance system
• Describe the key aspects of an outbreak
investigation
• Assess information on the effectiveness of
screening programs
• Assess the overall quality of a published
report of an epidemiological study

4. Unit – 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.

5. Assessment
• Sectional Test Week 9 25%
• Group
Assignment Week 12 25%
• Final Examination Week 15/16 50%
(2 hours)
All assessments must be completed and
submitted to pass the unit.

6. Text
• Gordis, L. Epidemiology. WB Saunders
Co., Philadelphia, 2004 (3rd Edition)
Other Readings

7. Contact details
Dr Aditi Dey
Room T337, Level 3, T Block
Phone 9351 9058, 9351 9494
Email A.Dey@fhs.usyd.edu.au

8. Lecture Outline
• Introduction
• Population Health & Disease Measures
• Causation and Epidemiological Study
Types
• Cohorts and Case- Control Studies and
measures of association
• Randomised Controlled Trials
• Ecological studies and Cross Sectional
Surveys

9. Lecture Outline
• Sources of Error:
– Selection & Measurement
• Confounding
• Health Surveillance
• Disease Outbreaks
• Critical Appraisal
• Screening
• Using Epidemiology to Evaluate Health
Services/Review

10. Learning Objectives for today
• Define epidemiology
• List the uses of epidemiology
• Describe key historical epidemiological
studies
• Explain the key features of
epidemiological studies

11. Origin of the term ‘epidemiology’
• epi - ‘on, upon, at, by, near, over, on top
of, against, among’
• demos - ‘common people or citizenry’
• ology - ‘the study of’
• epidemiology =‘Study of disease among
the population’

12. Definition of Epidemiology
• ‘study of the distribution and determinants
of health related states or events in
specified populations and the application
of this study to the control of health
problems’
Last, 1995

13. Epidemiology is about
Populations
• Groups of people not individuals
• It answers population questions
– aetiology of disease
– prevention of disease
– Extent/distribution of disease (allocation of
effort & resources in health facilities and
communities)

14. Epidemiology and Clinical Practice
Practice of medicine is dependent on population
data. For example:
• A physician hears a apical systolic murmur and
knows it represents mitral regurgitation.
• How does this knowledge originate?
• The diagnosis is based on association of
auscultatory findings with the findings of surgical
pathology or autopsy in a large group of
patients. Hence, diagnosis is population based

15. Example: Prognosis
A patient may ask a doctor, “How long will I
live?’’
The doctor usually answers on the basis of :
• Experience with large groups of patients
who had the same disease
• Were observed at the same stage of
disease and received the same treatment

16. Relationship between
Epidemiology and Clinical Medicine
Studies/Assessments Diagnosis
Prevention Treatment
Evaluation Cure
Planning Care

17. Examples of Epidemiological Studies
(Use of observational data for prevention of
disease)
Vaccination: Prevention of smallpox
• Edward Jenner observed that dairy maids (women
who milked cows) developed a mild disease called
cowpox.
• Later, in outbreaks of smallpox, these dairy maids
did not develop smallpox and overheard one of
them say, “I can’t take smallpox for I already had
the cowpox.’’ This information was observational.
• Jenner decided to test the hypothesis that cowpox
could provide protection against smallpox.

18. Examples of Epidemiological
Studies
• Vaccination –
Edward Jenner &
the smallpox
vaccination

19. First 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

20. • Jenner knew nothing about the biology of
the disease or about viruses
• He based his hypothesis from purely
observational data

21. Examples of Epidemiological
Studies
• 1854 Outbreak of Cholera in London: sign
at cemetery in Dudley, England

22. Epidemiology of cholera
• John Snow and epidemiology of cholera
• John Snow: anaesthesiologist who
administered chloroform to Queen Victoria
in childbirth
• In the 1st week of September, 1854 about
600 people died from cholera.
• These people lived within a few blocks of
the Broad Street pump in London

23. Cholera outbreak - map

24. • John Snow believed that cholera was caused by
contaminated water (others had other theories:
miasmatic theory)
• In London at that time, a person obtained water by
signing up with one of the water supply companies.
• The intake for the water companies was a very polluted
part of Thames river.
• One of the companies (Lambert Company) shifted its
water intake upstream in the Thames to a less polluted
part of the river but the others didn’t.
• Snow hypothesised that: mortality from cholera would
be lower in people getting water from Lambert
Company than those getting water from other
companies

25. Rates of cholera deaths
Water Supply Cholera Deaths per
10,000 houses
Southwalk & 315
Vauxhall
Lambeth Co 38
Other districts in 56
London

26. The 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.

27. Examples of Epidemiological
Studies
• Link between smoking and lung cancer
Doll & Hill, 1964

28. Examples of Epidemiological Studies
Water
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

29. Trial in New York communities:
Newburg and Kingston
• DMF (decayed, missed and filled) index
was used.
• Baseline information from both
communities collected
• Water in Newburg was fluoridated and
children re-examined
• DMF index in Newburg had dropped after
10 years

30. Rate of decayed teeth – regional
differences

31. • Water fluoridation is a controversial issue
so after fluoride was added to its water
supply, it was discontinued after a
referendum in Wisconsin (USA)
• The next figure shows that after fluoride
was removed, the DMF index rose
• This provided further evidence that
fluoride acted to prevent dental caries

32. Decayed teeth – after
fluoridisation

33. Epidemiological Questions
• When can we expect the next flu
epidemic?
• Are the number of AIDS cases increasing
or decreasing?
• Should we screen the male population for
prostate cancer?
• How can cervical cancer best be
prevented?
• Has the slip, slap, slop campaign reduced
skin cancers rates?

34. Uses of Epidemiology(Gordis,
2000)
• Identifies aetiology or causes of disease
including the risk factors for the disease.
• Determine the extent of the disease in the
community
• Examines natural history of disease and
prognosis of disease

35. Uses of Epidemiology(Gordis,
2000)
• Describes and monitors the population health
and the patterns of disease
• Evaluates new preventive and therapeutic
interventions and modes of health care delivery
• Provides information to inform public policy
decisions

36. Identifies aetiology or causes of
disease including the risk factors for the
disease.
Doll and Hill,1964

37. Determine the extent of the disease
in the community
NSW Health Survey,1997

38. Examines natural history of disease
and prognosis of disease Figure 1: The Natural History of NIDDM
Modifiable:
Modifiable: Modifiable:
Poor glucose control
Risk Inactivity Undiagnosed onset of complications
Inactivity
Factors Obesity Untreated progression of complications
Obesity
Non-Modifiable: Poor glucose control
Fat Intake
Genetics Hypertension
Hypertension
Age Dyslipidaemia
Dyslipidaemia
Poor Footcare
Poor Footcare
Smoking
Smoking
Non-Modifiable:
Non-Modifiable:
Genetics
Genetics
Disease Duration
Healthy Development of
Onset of complications
Natural History Population End-Stage
of NIDDM Onset of NIDDM Complications
(Often asymptomatic) (Blindness, ESRF, Heart
Attack, Amputations)
Population at
Risk
Ongoing
Primary Manage NIDDM Treat Complications Rehabilitation and
Screening for
Prevention Glucose Control Glucose Control Palliation
complications
Increase Activity Education Education Dialysis
Eyes
Improve Nutrition Increase Activity Increase Activity Post Amputation
Feet
Weight Control Reduce Obesity Reduce Obesity Rehab
CVD
Possible Diet Modification
Kidneys
Treat Hypertension Cardiac Rehab
Treat Hypertension Treat Hyperlipidaemia Stroke Rehab
Intervention Points Treat Hyperlipidaemia Treat Retinopathy Aids
Screen for NIDDM Self care Treat Vascular Problems Support
in at risk populations Reduce Smoking Self care
Reduce Smoking

39. Investigates and controls disease
outbreaks

40. Describes and monitors the population
health and the patterns of disease
NSW Cancer Registry, 2002

41. Describes and monitors the population
health and the patterns of disease –
breast cancer
NSW Cancer
Registry,
2002

42. Evaluates new preventive and
therapeutic interventions and modes 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?

43. Provides information to inform
public policy decisions - Smoking in
restaurants Smokefree Environment Bill, 2000
NSW Health Survey,1997

44. Determining Causation
• Disease has been classically described as
a result of an epidemiological triad
• It is the product of an interaction of the
human host, an agent (eg infectious) and
the environment that promotes the
exposure
• For such an interaction to occur, the host
must be susceptible

45. Determining Causation Agent
• Agent(s)
eg microorganisms,
chemicals, Disease
psychological
factors
Host Environment
• Environment Factors
physical, social or biological environment
circumstances
• Host Factors – susceptibility eg immunity,
behaviours
• Vectors of diseases: insects such as
mosquitoes, arthropods (ticks) etc

46. Determining Causation -
Infection Agent: Beta-haemolytic
streptococcus
Disease
Host: Environment
Low immunity Poverty, climate
Disease: Acute rheumatic fever
(polyarthritis, subcutaneous nodules over

47. Determining Causation - Chronic
Disease
- can be more complex
- usually the result of many factors (referred
to as risk factors) acting in sequence, or
together that result in disease in an
individual.

48. Causal Pathway or Causal Web
Source: Stallones, R.A. (1966)

49. Risk Factor is:
• An aspect of
– personal behaviour or
– Lifestyle or
– environmental exposure or
– an inborn or inherited characteristic
that is associated with an increased
occurrence of disease or other health-related
event or condition.

50. Risk Factor (major categories)
BEINGS model by Jekel et al 2001
• Biological & behavioural factors
• Environmental factors
• Immunological factors
• Nutritional factors
• Genetic factors
• Services, Social & Spiritual factors

51. Defining Populations in
Epidemiology
• Epidemiology focuses on defined
population and measures disease
outcomes & health in relation to a
population at risk
• Population at Risk = People, healthy or
sick have potential to develop a particular
health state or illness.
• They would be counted as cases if they
had the disease being studied

52. Example
• In a survey of hang-gliding accidents it was
recommended that flying should be banned
between 11am and 3pm because this was when
the greatest number (86%) of accidents
occurred
• During the month of May 1995:
– 200 hang-gliders flew between the hours of
11am and 3pm
– 50 accidents occurred during this time
– 10 people flew outside the hours of 11am &
3pm
– 8 accidents occurred during this time
• Given this information, determine what time you
are most likely to have an accident?

53. Example
• Accidents during 11am and 3pm
– 50 accidents per 200 people gliding
– 1 in 4 people = 25%
• Accidents outside the hours of 11am &
3pm
– 8 accidents 10 people flew occurred during
this time
– 4 in 5 people = 80%

54. Types of Populations
• Population at Risk = People, healthy
or sick who have the potential to
develop a particular health state
or illness.
• Populations may also be defined
as:
– cohort or
– dynamic populations

55. Cohort Populations
• Also called fixed populations
• Criteria for membership: experienced an
event or set of events
• Once the qualifying events have been
experienced by the person, membership of
the cohort becomes a permanent
• Examples:
– a birth cohort may consist of all people
born in 1973;
– people who have a tertiary degree.

56. Dynamic Population
• Have to meet certain criteria to be part
of the population
• Membership only lasts while the
qualifying criteria are present,
membership may be only temporary
• Examples:
– people attending university – membership
is temporary

57. Dynamic Population
• Can be in a steady state when a specific
feature remains constant despite changing
membership.
• Example:
– if 51% of a dynamic population is female and this
remains constant over time (eg some women die
and some women are born) - the population has a
steady state in respect to gender composition
• Are people in clinical trials a cohort or
dynamic population?

58. Key components of
epidemiological studies
Target
Population Exposure to a study
factor
Study Exposed
Population/ Outcome
Sample Unexposed

59. Key components of
epidemiological studies
• Target population is the population a
researcher wants to make
generalizations about
• Study population is the group a
researcher wishes to study (sometimes
the same as the target population)
• Study sample is a group of subjects
chosen for study to represent the study
population

60. Key components of
epidemiological studies
Target
Population Exposure to a study
factor
Study Exposed
Population/ Outcome
Sample Unexposed

61. Key components of
epidemiological studies
• Study factor
– is a element that is being investigated to see if it is
a determinant of a particular health problem
– or if it reduces the impact of a particular health
problem.
– Study factors can include
• risk factors for a health problem,
• interventions (therapeutic or
preventative) to ameliorate a health
condition,
• diagnostic tests or techniques and
• environmental exposures.

62. • Exposure is contact with or possessing a
particular study factor
• Exposed group is a group whose
members have had contact with or
possess a study factor

63. Key components of
epidemiological studies
• Unexposed group is a group that has
not had contact with a cause of, or
possess a characteristic that is a
determinant of, a particular health
problem.
• Outcome is any or all of the possible
results that may stem from an exposure
or study factor.

64. Conclusions Based on
Comparisons
• Clues to aetiology come from comparing
disease rates in groups with differing
levels of exposure
• Clues will be missed or false clues created
if comparisons are biased by unequal
collection of cases or exposure levels

65. Proportion of Deaths Attributable to Tobacco,
NSW, 1996-1998 (Close & Achat)
Cause of Death Western NSW
Sydney
Lung Cancer 31.3 29.3
Other cancer 8.5 9.0
Coronary heart 22.9 20.6
disease
Respiratory 24.9 27.7
disease

66. Next week
• Population Health and Disease Measures
– incidence
– prevalence
– death rates
– standardisation
Text: P31-59