chapter 5: introduction to study design · 2016. 1. 1. · 1. studies must address clearly defined...
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Epidemiology Kept Simple
Chapter 5:
Introduction to Study Design
Chapter 5
Intro to Epi Study Design
5.1 Introduction to Etiologic Research
5.2 Ethical Conduct of Studies in
Human Subjects
5.3 Selected Elements of Study Design
5.4 Common Types of Epidemiologic
Studies
Main types of epidemiologic studies
Observational Studies
Experimental Studies [Chapter 6]
Aggregate-level unit of observation
Person-level unit of observation
Cross-Sectional [Chapter 5]
Longitudinal
Community trials
Ecological [Chapter 4]
Field trials
Clinical trials
Cohort [Chapter 7]
Case-Control [Chapter 8]
We aim to understand the concepts
behind this study design taxonomy
Before considering epi study
design further, let us consider
epidemiologic hypotheses
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Epi Hypotheses1. Studies must address
clearly defined research
hypotheses (“research
questions”)
2. Hypotheses must based
on sound causal
mechanisms in specific
terms that can be tested
3. Hypotheses are
refinement as new
information becomes
available
Hypotheses are like nets;
only he who casts will catch.
-- Novalis
Skip to slide 10
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Selected elements of an epi
research question1. Source population
2. Exposure variable (E)
3. Disease variable (D)
4. Extraneous “confounding” variables (C1, C2,…, Ck)
5. Dose-response
6. Time-response
7. Sample size
Variables
“Yaz” example
E → D
C1─┘
C2─┘
⁞ ⁞
Ck─┘
Example: Does the progestin
in Yaz (DRSP) cause more
venous thromboembolism
(VTE) than 2nd generation
progestins?
Research question in
general terms: Does
exposure E cause disease D
after considering the
contributions of “potential
confounders” C1, C2, …, Ck?
DRSP → VTE
Age ────┘
Trauma──┘
⁞ ⁞
Surgery──┘
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Ethics: Studying Human
Subjects • The Belmont Report
– Respect for
individuals
– Beneficence
– Justice
• IRB oversight
• Data Safety Monitoring
Board (DSMB)
• Informed consent
• Equipoise
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Equipoise • Equipoise ≡
balanced doubt
• Cannot knowingly expose a participant to harm
• Cannot withhold known benefit to study subjects
• What’s left? (ANS: equipoise)
Is equipoise the over-riding
principles of trial ethics?
Scientific Ethics• Ethics = the set of standardized
rules for conducting yourself: like the tax code people are looking for loopholes and shelters while still staying within the rules
• Scientific ethics is beyond “regular” ethics; scientific ethics is “a bending over backwards to obtain intellectual honesty” – “I cannot give any scientist of any age
any better advice than this: The intensity of the conviction that a hypothesis is true has no bearing on whether it is true or not.” (Peter Medewar)
Peter Medewar
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Advocacy vs. Scientific Ethics
• The term “advocacy” is used in the general sense as applied to public health, commercial, political, and other types of interests planned with the end result in mind.”
• Scientific ethics: A bending over backwards to prove oneself wrong.
“I cannot give any scientist of any age any better advice than this: The intensity of the conviction that a hypothesis is true has no bearing on whether it is true or not.”
Sir Peter Medewar
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Selected Study Design
Elements
• Referent group
• Experimentation
• Unit of observation
• “Longitudinalness”
• Case-control vs.
cohort
The Referent Group
• We can only judge the effects of an
exposure relative to the baseline
determined by a referent or “control”group
• “The question of course is not whether
there is an association in the minds of any
particular individuals. The question is
whether the specific exposure contributed
to the causal mechanism of the disease”
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Experimentation
• Question. Does vitamin C (the exposure) prevent colds (the disease outcome)?
• Experimental Study. Assign vitamin C supplementation to some subjects (E+) and gives others a placebo (E-). Compare cold rates in the groups.
• Observational Study. Classifypeople into those who take vitamin C (E+) and those who don’t (E−). Compare cold rates in the groups.
Did the protocol assign the exposure?
Randomized Experiment
Observational cohort
Randomized Experiment vs.
Observational Cohort
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Unit of Observation
• Question. Does cigarettes smoking (the exposure) cause lung cancer (the disease outcome)?
• Person-level data. Classify individuals as smokers or non-smokers. Assess & compare rates of LungCA in exposed and nonexposed groups.
• Aggregate-level data. Classify level of smoking in various regions. Assess & compare rates of LungCA according to regional smoking rates.
Are data available on individuals?
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Longitudinal v X-sectional
• Question. Does exercise (the exposure) prevent diabetes (the disease outcome)?
• Longitudinal measurement. Identify exercisers (E+) & non-exercisers (E−) track individuals experience over time to assess and compare incidence rates of diabetes.
• Cross-sectional measurement. Assess current exercise habits to assess and compare prevalences of diabetes.
Can you track individual experience over time?
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Case-Control vs. Cohort
• Hypothesis. Does cigarette smoking cause lung cancer?
• Cohort. Identify smokers (E+) and non-smokers (E-) assess and compare lung cancer rates
• Case-control sample. Identify lung cancer cases (D+) and non-cases (D-) assess and compare smoking histories (E+/E-)
Selection of subjects based on exposure
(cohort) or disease (case-control) status?
odds of prior exposures
Illustrative Example 5.6: Toxic Shock and Continuous Tampon Use
• 44 cases and 44 friend controls
• 42 (95%) of the 44 cases used tampons
continuously throughout menstruation
• 34 (77%) of 44 controls did similarly
• cases more likely to use tampons
continuously
• Eventually lead to the discovery that Rely
brand tampons were too absorbent
(c) B. Gerstman 2007 Chapter 9 21
Major types of epidemiologic studies
Observational Studies
Experimental Studies [Chapter 6]
Aggregate-level unit of observation
Person-level unit of observation
Cross-Sectional [Chapter 5]
Longitudinal
Community trials
Ecological [Chapter 4]
Field trials
Clinical trials
Cohort [Chapter 7]
Case-Control [Chapter 8]
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TaxonomyI. Experimental studies (“trials”) – Ch 6
II. Observational studies
A. Individual-level data
1. Longitudinal
(a) Cohort studies - Ch 7
(b) Case-control studies – Ch 8
2. Cross-sectional – Ch 5
B. Aggregate-level data Ecological – Ch 4
Exercise 5.1 Instructions
• Clearly state the research question
• Identify the study exposure
• Identify the study outcome
• Determine the type of study according to
our taxonomy
Exercise 5.1A
Epidemiologists suspect that avian adeno-associated
virus is caused by exposure to poultry. Serum samples
from poultry workers and the general population are
tested to determine the proportion of individuals positive
for avian A–V antibody in each group.
1) experiment or observational
2) individual or aggregate unit of observation
3) longitudinal or X-sectional observation
4) cohort v case-control sample?
Type of study = x-sectional observational study
Exercise 5.1BThe behavioral pattern identified as Type A behavior is characterized by a
hard-driving personality susceptible to anger and time urgency. This type of
behavior is thought to be associated with increased risk for coronary heart
disease. Type A behavior is ascertained in a group of men in a postcoronary
disease rehabilitation program. Men not falling into the Type A category are
classified as Type B. Type A and Type B men are then followed for 5 years to
assess for the recurrence of acute coronary symptoms..
1) observational
2) individual
3) longitudinal
4) cohort
Type of study = observational cohort study
Exerise 5.1G
One hundred incident cases of infectious hepatitis…
1) observational
2) individual-
3) longitudinal
4) case-control sample?
Type of study = case-control!
Exerise 5.1H
Questionnaires are mailed to every tenth person listed…
1) observational
2) individual
3) X-sectional observation
4)
Type of study = x-sectional survey
Exerise 5.1I
An investigator collects information on the size of
manufacturing plants..
1) observational
2) aggregate
Type of study = ecological study!
Exerise 5.2
You have developed a hypothesis that automobile drivers
that talk on their cell phones….
(A) How might you measure the exposure in a cohort
study? Heavy talkers, Non-talkers
(B) How might you identify cases in this [cohort] study
(C)What additional factors…?
(D)What difficulties might be encountered when
measuring
(E) How would we design a case-control study to test…