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Types of study designs
Arash NajimiArash Najimi
PhD. CandidatePhD. CandidateDepartment of health education & health Department of health education & health
promotionpromotionIsfahan University of Medical SciencesIsfahan University of Medical Sciences
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Types of Studies
Descriptive Studies
Observational Analytic Studies Cross Sectional studies Ecologic studies Case Control studies Cohort studies
Experimental Studies Randomized controlled trials
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Hierarchy of Study Types
Descriptive•Case report•Case series•Survey
Analytic
Observational•Cross sectional•Ecologic•Case-control•Cohort studies
Experimental•Randomized controlled trials•Field Trials•Community Trials
Strength of evidence for causality between a risk factor and outcome
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Descriptive studies
Getting a “lay of the land” Surveys (NHIS, MCBS)
Describing a novel phenomena Case reports or case series
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Descriptive studies Cannot establish causal relationships
Still play an important role in describing trends and generating hypotheses about novel associations
The start of HIV/AIDS research Squamous cell carcinoma in sexual partner of Kaposi
sarcoma patient. Lancet. 1982 Jan 30;1(8266):286. New outbreak of oral tumors, malignancies and infectious
diseases strikes young male homosexuals. CDA J. 1982 Mar;10(3):39-42.
AIDS in the "gay" areas of San Francisco. Lancet. 1983 Apr 23;1(8330):923-4.
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Analytic Studies 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
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Hierarchy of Study Types
Descriptive•Case report•Case series•Survey
Analytic
Observational•Cross sectional•Ecologic•Case-control•Cohort studies
Strength of evidence for causality between a risk factor and outcome
Experimental•Randomized controlled trials•Field Trials•Community Trials
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Cross-sectional Study: Pluses
+ Prevalence (not incidence)
+ Fast/Inexpensive - no waiting!
+ No loss to follow up
+ Associations can be studied
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Cross-sectional study: minuses
time
- Cannot determine causality
Depression
Cigarette smoking
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Cross-sectional study: minuses
- Cannot determine causality
- Cannot study rare outcomes
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Case control studies
Investigator works “backward” (from outcome to predictor)
Sample chosen on the basis of outcome (cases), plus comparison group (controls)
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Study multiple exposures in a Case-control Study
Disease No Disease
CB
Exposed to A
CB
Not Exposedto A
CB
Exposed to A
CB
Not Exposedto A
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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: pluses
+ Rare outcome/Long latent period
+ Inexpensive and efficient: may be only feasible option
+ Establishes association (Odds ratio)
+ Useful for generating hypotheses (multiple risk factors can be explored)
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Case-control study-minuses
- Causality still difficult to establish
- Selection bias (appropriate controls)
- Recall bias: sampling (retrospective)
- Cannot tell about incidence or prevalence
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Cohort studies
• A cohort (follow-up, longitudinal) study is a comparative, observational study in which subjects are grouped by their exposure status, i.e., whether or not the subject was exposed to a suspected risk factor
• The subjects, exposed and unexposed to the risk factor, are followed forward in time to determine if one or more new outcomes (diseases) occur• Subjects should not have outcome variable on entry
• The rates of disease incidence among the exposed and unexposed groups are determined and compared.
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Study multiple outcomes in a cohort Study
Exposed Not Exposed
CB
DevelopDisease A
CB
Do not Develop
Disease A
CB
DevelopDisease A
CB
Do not Develop
Disease A
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Elements of a cohort study
Selection of sample from population Measures predictor variables in sample Follow population for period of time Measure outcome variable
Famous cohort studies Framingham Nurses’ Health Study Physicians’ Health Study Olmsted County, Minnesota
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time
The present The future
Top USMLE scorers
Everyone else
Prospective cohort study structure
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Strengths of cohort studies Know that predictor variable was present
before outcome variable occurred (some evidence of causality)
Directly measure incidence of a disease outcome
Can study multiple outcomes of a single exposure (RR is measure of association)
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Weaknesses of cohort studies Expensive and inefficient for studying rare
outcomes
Often need long follow-up period or a very large population
Loss to follow-up can affect validity of findings
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Other types of cohort studies Retrospective cohort
Identification of cohort, measurement of predictor variables, follow-up and measurement of outcomes have all occurred in the past
Much less costly than prospective cohorts Investigator has minimal control over study
design
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Other types of cohort studies Nested case-control study
Case-control study embedded in a cohort study Controls are drawn randomly from study sample
Case cohort Study
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Hierarchy of Study Types
Descriptive•Case report•Case series•Survey
Analytic
Observational•Cross sectional•Ecologic•Case-control•Cohort studies
Strength of evidence for causality between a risk factor and outcome
Experimental•Randomized controlled trials•Field Trials•Community Trials
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Randomized controlled trials
Investigator controls the predictor variable (intervention or treatment)
Major advantage over observational studies is ability to demonstrate causality
Randomization controls unmeasured confounding
Only for mature research questions
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Basic Trial Design
PopulationPopulation
SampleSample
TreatmentTreatment Dx No Dx
ControlControl Dx No DxPlaceboPlacebo
RandomizationRandomization
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Steps in a randomized controlled trial
1. Select participants2. Measure baseline variables3. Randomize
Eliminates baseline confounding Types (simple, stratified, block)
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Steps in a randomized controlled trial
4. Blinding the intervention As important as randomization
5. Follow subjects6. Measure outcome
Clinically important measures Adverse events
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Comparing Cohort Studies with Randomized Trials
Interventional Study Observational Study
Study group Study group
Random Allocation No Allocation
Group A Group B Group A Group B
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Hierarchy of Study Types
Descriptive•Case report•Case series•Survey
Analytic
Observational•Cross sectional•Ecologic•Case-control•Cohort studies
Strength of evidence for causality between a risk factor and outcome
Experimental•Randomized controlled trials•Field Trials•Community Trials