epidemiology lecture 1
TRANSCRIPT
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Epidemiologic Study Epidemiologic Study DesignsDesigns
Acknowledgements:
M. Tevfik DORAK
Ahmed MandilAhmed Mandil
Kimberly R. Barber
Birgit Greiner
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Study design: Definition
A study design is a specific plan or protocol for conducting the study, which allows the investigator to translate the conceptual hypothesis into an operational one.
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Epidemiologic Study Designs
Experimental Observational
DescriptiveAnalytical
Case-Control Cohort+ cross-sectional & ecologic
(RCTs)
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Descriptive studiesDescriptive studies
Examine patterns of disease
Analytical studiesAnalytical studies
Studies of suspected causes of diseases
Experimental studiesExperimental studies
Compare treatment modalities
Epidemiologic Study Designs
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Epidemiologic Study Designs
Grimes & Schulz, 2002
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Hierarchy of Epidemiologic Study DesignHierarchy of Epidemiologic Study Design
Tower & Spector, 2007
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ObservationalObservational Studies Studies
(no control over the circumstances)
- Descriptive: Most basic demographic studies
* Case Report
* Case Series
* Cross sectional
* Ecological/Correlation study
- Analytical: Comparative studies testing an hypothesis * cross-sectional (a snapshot; no idea on cause-and-effect relationship)
* cohort (prospective; cause-and-effect relationship can be inferred)
* case-control (retrospective; cause-and-effect relationship can be inferred)
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Epidemiologic Study Designs
Grimes & Schulz, 2002
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Case Report• What?
the profile of a single patient is reported in detail by one or more clinicians
• Example
In 1961, a published case report of a 40 year-old women who developed pulmonary embolism after beginning use oral contraceptive
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Case Series• What?
An individual case report that has been expanded to include a number of patients with a given disease
• Example
In Los Angeles, five young homosexuals men, previously healthy, were diagnosed with pneumocyst cariini pneumonia in a 6-month period
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Ecological or Correlation
• Ecological Studies– whole population is the unit of analysis– relationship between exposure and outcome at the
individual level is missing (incomplete design)
– ecological fallacy
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AnalyticalAnalytical Studies Studies
(comparative studies testing an hypothesis)
* cohort (prospective)
Begins with an exposure (smokers and non-smokers)
* case-control (retrospective)
Begins with outcome (cancer cases and healthy controls)
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PopulationPeople without disease
Exposed
Not exposed
Disease
No disease
Disease
No disease
Cohort StudiesCohort Studies
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Coh
ort D
esig
n
time
Study begins here
Studypopulation
free ofdisease
Factorpresent
Factorabsent
disease
no disease
disease
no disease
presentfuture
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Examples of Cohort StudiesExamples of Cohort Studies
*Framingham Heart Study (www)
* Physicians' Health Study (www)
* Nurses' Health Study (www)
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Advantages of Cohort StudiesAdvantages of Cohort Studies
- Can establish population-based incidence
- Accurate relative risk (risk ratio) estimation
- Can examine rare exposures (asbestos > lung cancer)
- Temporal relationship can be inferred (prospective design)
- Time-to-event analysis is possible
- Can be used where randomization is not possible
- Magnitude of a risk factor’s effect can be quantified
- Selection and information biases are decreased
- Multiple outcomes can be studied (smoking > lung cancer, COPD, larynx cancer)
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Disadvantages of Cohort StudiesDisadvantages of Cohort Studies
- Lengthy and expensive
- May require very large samples
- Not suitable for rare diseases
- Not suitable for diseases with long-latency
- Unexpected environmental changes may influence the association
- Nonresponse, migration and loss-to-follow-up biases
- Sampling, ascertainment and observer biases are still possible
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Population Cases (follow up 2 years)
HIV + 215 8HIV - 289 1
Presentation of cohort data: Presentation of cohort data: Population at riskPopulation at risk
Does HIV infection increase risk of developing TB among a population of drug users?
Source: Selwyn et al., New York, 1989EPIET
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Relative Risk calculation
Disease Un-disease
Expose a b a+b
Un-expose c d c+d
Incidence in expose (Ie)=a/a+b
Incidence in unexpose (Io)=c/c+d
Relative Risk=Ie/Io
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Exposure Population (f/u 2 years)
Cases Incidence
(%) Relative
Risk
HIV +
215
8
3.7
12.3
HIV - 298 1 0.3
Does HIV infection increase risk of developing TB Does HIV infection increase risk of developing TB among drug users?among drug users?
EPIET
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time
Exposure Study startsDisease
occurrence
Prospective cohort studyProspective cohort study
time
ExposureStudy startsDisease
occurrence
EPIET
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Retrospective cohort studiesRetrospective cohort studies
Exposure
time
Diseaseoccurrence Study starts
EPIET
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Cohort StudiesCohort Studies
Grimes & Schulz, 2002
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Population
Cases
Controls
Exposed
Case-Control StudiesCase-Control Studies
Not exposed
Exposed
Not exposed
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Case-Control StudiesCase-Control Studies
Schulz & Grimes, 2002
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Odds ratio calculation
Case Control
Expose a b
Un-expose c d
Odds Ratio =ad/bc
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Advantages of Case-Control StudiesAdvantages of Case-Control Studies
- Cheap, easy and quick studies
- Multiple exposures can be examined
- Rare diseases and diseases with long latency can be studied
- Suitable when randomization is unethical (alcohol and pregnancy outcome)
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Disadvantages of Case-Control StudiesDisadvantages of Case-Control Studies
- Case and control selection troublesome
- Subject to bias (selection, recall, misclassification)
- Direct incidence estimation is not possible
- Temporal relationship is not clear
- Multiple outcomes cannot be studied
-If the incidence of exposure is high, it is difficult to show the difference between cases and controls
-Reverse causation is a problem in interpretation
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Case-Control StudiesCase-Control Studies: :
Potential BiasPotential Bias
Schulz & Grimes, 2002
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Application Exercise:Case / Control Study
• Describe a case/control study on the relationship between childhood obesity, smoking history, and occurrence of hypertension in middle-aged men.
• What research question can we answer?
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Cross-Sectional Studies
• Measurement of risk and outcome at the same time.
Outcome
Risk factor
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Cross-Sectional Design
• The only study capable of calculating prevalence.– Proportion of the
population with the outcome at any point in time.
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Application Exercise: Cross-Sectional Study
• Design a cross-sectional study that examines the relationship between dietary sodium and hypertension in middle-aged men.
• What research question can we answer?
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Cross-Sectional StudiesAdvantages
• Cheap and quick studies.
• Data is frequently available through current records or statistics.
• Ideal for generating new hypothesis.
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Cross-Sectional StudiesDisadvantages
The importance of the relationship between the cause and the effect cannot be determined.
• Temporal weakness:– Cannot determine if cause preceded the effect
or the effect was responsible for the cause.– The rules of contributory cause cannot be
fulfilled.
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Type of Study
Alternative Name
Unit of Study
Randomised Controlled Trials
Field Trials
Community Trials
Clinical Trials
Community Intervention Studies
Patients
Healthy People
Communities
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Types of trials
B lind ed N o t b lind ed
R a nd o m ised N o t ran d om ised
C o n tro lled N o t co n tro lled
T ria l
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Exp
erim
enta
l Des
ign
timeStudy begins here (baseline point)
Studypopulation
Intervention
Control
outcome
no outcome
outcome
no outcome
baselinefuture
RANDOMIZATION
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Intervention Randomisation
Control Blinding
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Experimental studies are useful for evaluating:
• New drug or other treatment for disease• New medical/health care technology• Methods of prevention• Methods of health promotion• New health protection policies• Programs for screening and diagnosis• Methods of providing health care• New health care policies
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Ethical Considerations in experimental studies
• Is proposed treatment safe?• For the sake of trial, can a treatment ethically
be withheld?• What patients may be brought into trial and
allocated randomly to treatments?• Is it ethical to use a placebo or dummy
treatment?• Is it proper for the trial to be in any way
masked?Adapted from Hill (1977)
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Advantages (I)
– the “gold standard” of research designs. They thus provide the most convincing evidence of relationship between exposure and effect. Example:
• trials of hormone replacement therapy in menopausal women found no protection for heart disease, contradicting findings of prior observational studies
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Advantages (II)
• Best evidence study design• No selection bias (using blinding)• Controlling for possible confounders• Comparable Groups (using
randomization)
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Disadvantages
• Is the most expensive study design in terms of money, time, and number of patients.– Issues of patient attrition and compliance may
invalidate the results.– Can be problematic for ethical reasons.
• Use of placebo
• Harm outweighing benefits
• Zero tolerance for some exposures
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