Transcript Types of study designs: from cross

Types of study designs
Arash Najimi
PhD. Candidate
Department of health education & health promotion
Isfahan University of Medical Sciences
Types of Studies
 Descriptive Studies
 Observational Analytic Studies
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Cross Sectional studies
Ecologic studies
Case Control studies
Cohort studies
 Experimental Studies
 Randomized controlled trials
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
Descriptive studies
 Getting a “lay of the land”
 Surveys (NHIS, MCBS)
 Describing a novel phenomena
 Case reports or case series
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.
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
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
Cross-sectional Study: Pluses
+
Prevalence (not incidence)
+
Fast/Inexpensive - no waiting!
+
No loss to follow up
+
Associations can be studied
Cross-sectional study: minuses
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Cannot determine causality
Cigarette smoking
Depression
time
Cross-sectional study: minuses
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Cannot determine causality
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Cannot study rare outcomes
Case control studies
 Investigator works “backward”
(from outcome to predictor)
 Sample chosen on the basis of
outcome (cases), plus comparison
group (controls)
Study multiple exposures in a
Case-control Study
C
B
C
B
Exposed
to A
Not Exposed
to A
Disease
C
B
C
B
Exposed
to A
Not Exposed
to A
No Disease
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
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)
Case-control study-minuses
- Causality still difficult to establish
- Selection bias (appropriate controls)
- Recall bias: sampling (retrospective)
- Cannot tell about incidence or prevalence
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
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Subjects should not have outcome variable on entry
The rates of disease incidence among the
exposed and unexposed groups are
determined and compared.
Study multiple outcomes in a
cohort Study
Not Exposed
Exposed
Develop
Disease A
B
C
Do not
Develop
Disease A
Develop
Disease A
B
C
B
C
Do not
Develop
Disease A
B
C
Elements of a cohort study
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Selection of sample from population
Measures predictor variables in sample
Follow population for period of time
Measure outcome variable
 Famous cohort studies
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Framingham
Nurses’ Health Study
Physicians’ Health Study
Olmsted County, Minnesota
Prospective cohort study structure
The present
The future
Top USMLE scorers
Everyone else
time
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)
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
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
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
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
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
Basic Trial Design
Population
Sample
Treatment
Dx
No Dx
Randomization
Control
Placebo
Dx
No Dx
Steps in a randomized
controlled trial
1. Select participants
2. Measure baseline variables
3. Randomize
 Eliminates baseline confounding
 Types (simple, stratified, block)
Steps in a randomized
controlled trial
4. Blinding the intervention
 As important as randomization
5. Follow subjects
6. Measure outcome
 Clinically important measures
 Adverse events
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
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