dr. mehrnaz nikouyeh emergency physician. the study of the distribution and determinants of...
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Study design
Dr. Mehrnaz NikouyehEmergency physician
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The 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
Definitions of Research
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To describe the frequency and extent of health
conditions and disease. To determine the burden of disease in a community,
including socioeconomic impact of disease occurrence in specific populations.
To identify the causes and risk factors of specific diseases. This is the basis of disease prevention.
To evaluate medical interventions, including both preventive and therapeutic measures, and evaluate the delivery of these measures in health care settings.
To study the natural history and prognosis of disease. To provide the foundation for developing public policy
and regulatory decisions relating to health.
PURPOSE OF EPIDEMIOLOGY
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A good clinical study starts with a good question based on good
hypothesis that is based on good and comprehensive review of the available evidence from pre-clinical and clinical data
Type of design depends on the question to be answered
Where to Start?
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Focused and specific Supported by available data Not a replication of already
established evidence Ethical Answerable Methods, resources ….etc.
Formulating a Research Question
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Specific aims
Clear and detailed End point(s)
Primary The main answer to the research question
Secondary Answer other related questions
Objectives
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• Describe• Analyze
Your question
• Retrospective• Prospective
Your resources
• Acceptance of research• Observational• Interventional
Community
Study Design
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Descriptiv
e
Case repot
Case series Survey
Analytic
Observational
Cross sectinal
Case control
Cohort
Experimental
RCT
Clinical Study Types
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Types of primary studies
Descriptive studies describe occurrence of
outcome
Analytic studies describe association between
exposure and outcome
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10
Observational study Clinical trial
exposed
non exposedoutcome
ClinicalTrial
observationalstudydescribe as
occurring in nature
allocaterandomly
Ethics!
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Hierarchy of Epidemiologic Study Design
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Validity: Truth
External Validity: Can the study be generalized to the
population Internal Validity:
Results will not be due to chance, bias or confounding factors
Symmetry Principle: Groups are similar
Important issues in Study Design
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13
Confounding: distortion of the effect of one risk
factor by the presence of another Bias: Any effect from design, execution, &
interpretation that shifts or influences results Confounding bias: failure to account for the
effect of one or more variables that are not distributed equally
Measurement bias: measurement methods differ between groups
Sampling (selection) bias: design and execution errors in sampling
Important issues in Study Design
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Descriptive studies
Cannot establish causal relationships Still play an important role in
describing trends and generating hypotheses about novel association
Describing a novel phenomena
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Attempt to establish a causal link between
a predictor/risk factor and an outcome.
You are doing an analytic study if you have any of the following words in your research question: greater than, less than, causes, leads to,
compared with, more likely than, associated with, related to, similar to, correlated with
Analytic Studies
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Timeframe of Studies
Prospective Study - looks forward, looks to the future, examines future events, follows a condition, concern or disease into the future
time
Study begins here
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Timeframe of Studies
Retrospective Study - “to look back”, looks back in time to study events that have already occurred
time
Study begins here
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Descriptive Studies
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Case Reports
Detailed presentation of a single case or handful of cases
Generally report a new or unique finding
e.g. previous undescribed disease e.g. unexpected link between diseases e.g. unexpected new therapeutic effect e .g. adverse events
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Case Series
Experience of a group of patients with a similar diagnosis
Assesses prevalent disease Cases may be identified from a single or
multiple sources Generally report on new/unique condition May be only realistic design for rare
disorders
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Case Series
Advantages Useful for hypothesis generation Informative for very rare disease with few
established risk factors Characterizes averages for disorder
Disadvantages Cannot study cause and effect relationships Cannot assess disease frequency
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Case Report
Case Series
DescriptiveEpidemiology Study
One case of unusualfindings
Multiple cases of findings
Population-based cases with denominator
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Analytical Studies
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Observational Studies
non-experimental observational because there is no individual intervention treatment and exposures occur in a “non-controlled”
environment individuals can be observed prospectively, retrospectively,
or currently Possibility of confounding No control over study units
need to clearly describe study individuals Can study risk factors that have serious consequences Study individuals in their natural environment (>>
extrapolation)
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cohort
Case control
Cross sectional
Types of observational studies
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Evaluate the effect of a suspected risk
factor (exposure) on an outcome (e.g. disease) define “exposure” and “disease”
Describe the impact of the risk factor on the frequency of disease in a population
Aims of observational studies
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Temporal relations of observational studies
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Cross - Sectional Study
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Cross-sectional studies
An “observational” design that surveys exposures and disease status at a single point in time (a cross-section of the population)
time
Study only exists at this point in time
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Cross-sectional Design
time
Study only exists at this point in time
Studypopulation
No Disease
Disease
factor present
factor absent
factor present
factor absent
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Cross-sectional Studies
Often used to study conditions that are relatively frequent with long duration of expression (nonfatal, chronic conditions)
It measures prevalence, not incidence of disease. Comparison of prevalence among exposed and non-exposed.
Not suitable for studying rare or highly fatal diseases or a disease with short duration of expression
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Cross - Sectional Study
Random sample from population i.e. results reflect reference population
Estimates the frequencies of both exposure and outcome in the population
Measuring both exposure and outcome at one point in time
Typically a survey
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Cross - Sectional Study advantages
Can study several exposure factors and outcomes simultaneously
Determines disease prevalence Helpful in public health administration & planning Quick Low cost (e.g. mail survey) Very useful for public health planning Disease etiology. Conduct this by obtaining data on
risk factors for a disease. Hypothesis generating
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Cross-sectional studies disadvantages
Weakest observational design, (it measures prevalence, not incidence of disease). Prevalent cases are survivors
The temporal sequence of exposure and effect may be difficult or impossible to determine
Usually don’t know when disease occurred Rare events a problem. Quickly emerging
diseases a problem Does not determine causal relationship
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Cross-Sectional: Pediatrician-to-Child Ratio
Greg et al. (2001) Pediatrics.107(2):e18
0
5
10
15
20
25
30
35
40
Ped
iatr
icia
ns p
er
1000
Chi
ldre
n
Rural Urban
1981198619911996
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Case-Control Studies
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Case-Control Study Design
Cases
Controls
Exposed
Unexposed
Exposed
Unexposed
TimeData
collectionDirection of inquiry
Q: What happened?
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Case control studies
Determines the strength of the association between each predictor variable and the presence or absence of disease
Cannot yield estimates of incidence or prevalence of disease in the population (why?)
Odds Ratio is statistics
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Case-Control Study
Retrospective Can use hospital or health register data
First identify cases Then identify suitable controls
Hardest part: who is suitable ?? Critical that the exposure in the controls is representative
of the exposure in the population Ideal controls would have same/similar characteristics as
the cases Matching cases to controls
Then inquire or retrieve previous exposure By interview By databases (e.g. hospital, health insurance)
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Case-Control Study
Strengths Less expensive and time consuming
Efficient for studying rare diseases Can study many risk factors at the same time Smaller sample Multiple etiologic factors evaluated for single disease Good for diseases with long latency
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Limitations
Inappropriate when disease outcome for a specific exposure is not known at start of study
Exposure measurements taken after disease occurrence
Disease status can influence selection of subjects Confounding likely difficulties in selection of controls ascertainment of disease & exposure status inefficient for rare exposures unless attributable risk is
high
Case-Control Study
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Case-control study-minuses
- Causality still difficult to establish
- Selection bias (appropriate controls)- Caffeine and Pancreatic cancer in the GI
clinic
- Recall bias: sampling (retrospective)- Abortion and risk of breast cancer in
Sweden
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Case Selection
• Define source population• Cases
– incident/prevalent– diagnostic criteria (sensitivity + specificity)
• Controls– selected from same population as cases– select independent of exposure status
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Control Selection
• Random selection from source population• Hospital based controls:
– convenient selection– controls from variety of diagnostic groups other
than case diagnosis– avoid selection of diagnoses related to
particular risk factors– limit number of diagnoses in individuals
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Case-Crossover Studies
Study of “triggers” within an individual ”Case" and "control" component, but
information of both components will come from the same individual
”Case component" = hazard period which is the time period right before the disease or event onset
”Control component" = control period which is a specified time interval other than the hazard period
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Cohort Studies
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Prospective Cohort Study
without outcome
Cohort
with outcome
with outcome
withoutoutcome
Exposed
Unexposed
TimeOnsetof study Direction of inquiry
Q: What will happen?
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Prospective Cohort study
Measure exposureand confounder
variables
Exposed
Non-exposed
Outcome
OutcomeBaseline
time
Study begins here
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Retrospective Cohort
study
Measure exposureand confounder
variables
Exposed
Non-exposed
Outcome
OutcomeBaseline
time
Study begins here
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Cohort studies
Follow-up studies; subjects selected on presence or absence of exposure & absence of disease at one point in time. Disease is then assessed for all subjects at another point in time.
Typically prospective but can be retrospective, depending on temporal relationship between study initiation & occurrence of disease.
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Cohort studies
1. Prospective cohort study – outcome event of interest occur after the study is initiated
2. Retrospective cohort study – outcome events of interest have already occurred at study initiation Can be regarded as a reconstruction of a cohort
study which has already taken place Generally cheaper and faster than a prospective
study Requires good historical exposure data on subjects
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Cohort studies
More clearly established temporal sequence between exposure & disease
Can establish population-based incidence Accurate relative risk (risk ratio) estimation Can examine rare exposures (asbestos > lung
cancer) Can be used where randomization is not possible Allows direct measurement of incidence Examines multiple effects of a single exposure - Magnitude of a risk factor’s effect can be quantified Selection and information biases are decreased
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Cohort Study
Strengths Exposure status determined before disease detection Subjects selected before disease detection Can study several outcomes for each exposure Directly measure incidence of a disease outcome
Limitations Expensive and time-consuming Inefficient for rare diseases or diseases with long latency Loss to follow-up and unavailability of data potential confounding factors Unexpected environmental changes may influence the association
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Basic Question in Analytic Epidemiology
Are exposure and disease linked?
Exposure Disease
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Experimental Studies
In an experiment, we are interested in the consequences of some treatment on some outcome.
The subjects in the study who actually receive the treatment of interest are called the treatment group.
The subjects in the study who receive no treatment or a different treatment are called the comparison group.
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Epidemiologic Study Designs
Randomized Controlled Trials (RCTs) a design with subjects randomly assigned to
“treatment” and “comparison” groups
provides most convincing evidence of relationship between exposure and effect
not possible to use RCTs to test effects of exposures that are expected to be harmful, for ethical reasons
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Clinical Trials
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Uncontrolled Controlled
Before/after (cross-over) Historical Concurrent, not randomized Randomized
Clinical Trial: Study Design
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May Be AppropriateEarly studies of new and untried therapies
Uncontrolled early phase studies where the standard is relatively ineffective
Investigations which cannot be done within the current climate of controversy
Truly dramatic response
Non-randomized Trials
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Randomized controled trial
timeStudy begins here (baseline point)
Studypopulation
Intervention
Control
outcome
no outcome
outcome
no outcome
baseline future
RANDOMIZATION
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Epidemiologic Study
Designs
Randomized Controlled Trials (RCTs) the “gold standard” of research designs provides most convincing evidence of
relationship between exposure and effect
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 of Randomized Control Clinical Trial
advantages Randomization tends to produce
comparable groups Assure causal relationship Randomization produces valid statistical
tests
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Disadvantages of Randomized Control Clinical Trial
1. Generalizable Results? Participants studied may not represent
general study population.
2. Recruitment Hard
3. Acceptability of Randomization Process Some physicians will refuse Some participants will refuse
4. Administrative Complexity5. Very expensive
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Clinical Trials-Phases
Phase I - Does it hurt the Patient? Usually in normal volunteers, small groups for
safety testing
Phase II - Does it help the Patient? On patients to confirm the effectiveness of the
drug
Phase III - Is it any better? Large groups of patients for statistical confirmation of effect
and incidence of side-effects
Phase IV - Does it work in the community? Post marketing studies. Fine tuning and new
rare findings from a very large population