2,3 types of study designs
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Dr. Wafaa El-Kassed
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To understand the difference betweendescriptive and analytic studies
To identify the hierarchy of study designs, andthe strengths and weakness of each design
Dr. Wafaa El-Kassed
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Descriptive Studies: Case Report
Case Series
Surveillance
Analytic Studies:
Observational
Cross Sectional studies
Case Control studies Cohort studies
Experimental Studies
Randomized controlled trialsDr. Wafaa El-Kassed
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Descriptive
Case report
Case series
Survey
Analytic
Observational
Cross sectional
Case-controlCohort studies
Experimental
Randomized
controlled trials
Strength of evidence for causality between a risk factor and outcomeDr. Wafaa El-Kassed
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Purpose:
To describe a phenomenon
To explore factors that influence and interact with it
Descriptive Research
Document conditions, attitudes, or characteristicsof individuals or groups.
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Cannot establish causal relationships
Still play an important role indescribing trends and generatinghypotheses about novel associations
Ex. The start of HIV/AIDS research
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A hypothesis is an educated guess about an
association that is testable in a scientific
investigation.
Descriptive data provide information to
develop hypotheses
Hypotheses tend to be broad initially and are
then refined to have a narrower focus
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The case report is the least publishable unit inthe medical literature.
Often, an observant clinician reports anunusual disease or association, which prompts
further investigations with more rigorous study
designs.
For example, a clinician, reported benign
hepatocellular adenomas, a rare tumour, in
women who had taken oral contraceptives.Dr. Wafaa El-Kassed
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Not all case reports deal with serious
health threats, however; some simply
enliven the generally drab medicalliterature.
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A case-series aggregates individual cases inone report.
Sometimes, the appearance of several similar
cases in a short period heralds an epidemic.
For example: a cluster of homosexual men in
Los Angeles with a similar clinical syndromealerted the medical community to the AIDS
epidemic in North America.
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Surveillance is an important type ofdescriptive study.
Surveillance can be thought of aswatchfulness over a community.
A more formal definition is the ongoing
systematic collection, analysis, and
interpretation of health data essential to theplanning, implementation, and evaluation of
public health practiceDr. Wafaa El-Kassed
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Inexpensive as data are already available.
Efficient to use.
Few ethical difficulties exist.
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Temporal associations between putative
causes and effects might be unclear.
A dangerous pitfall is that the investigatorsmight draw causal inferences when none is
possible.
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Descriptive studies are often the first, tentative
approach to a new event or condition.
These studies generally emphasize features ofanew disease or assess the health status of
communities. Health administrators use descriptive studies
to monitor trends and plan for resources.
By contrast, epidemiologists and cliniciansgenerally use descriptive reports to search for
clues of cause of disease
ie, generation of hypotheses.
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Descriptive
Case report
Case series
Survey
Analytic
Observational
Cross sectional
Case-controlCohort studies
Experimental
Randomized
controlled trials
Strength of evidence for causality between a risk factor and outcomeDr. Wafaa El-Kassed
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Attempt to establish a causallinkbetweena predictor/risk factor and an outcome.
You are doing an analytic study if youhave 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 withDr. Wafaa El-Kassed
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Two main categories:1. Experimental
2. Observational
1. Experimental studies exposure status isassigned
2. Observationalstudies exposure statusis not assigned
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Can involve individuals or communities
The non-exposed group can be untreated(placebo) or given a standard treatment
Most common is a randomized clinical trial
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Randomized clinical trial to determine if givingmagnesium sulfate to pregnant women inpreterm labor decreases the risk of their
babies developing cerebral palsy
Randomized community trial to determine if
fluoridation of the public water supply
decreases dental cavities
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It is the gold standard by which the otherdesigns must be judged.
Investigator controls the predictor variable(intervention ortreatment)
Major advantage over observational studies is
ability to demonstrate causality
Randomization controls unmeasured
confounding. Only for mature research questions
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PopulationPopulation
SampleSample
Treatment
Placebo
RandomizationRandomizationRandomizationRandomization
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1. Select participants
2. Measure baseline variables
3. Randomize the groups
4. Blinding administration
5. Follow up subjects Adherence to protocol
6. Measure outcome Clinically important measures
Adverse events. Dr. Wafaa El-Kassed
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Randomization is of central importance in
clinical trials.
Ingeneral, a randomized trial is an essential
toolfor testing the efficacy of the treatment.
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The basic benefits of randomization include:
1. Eliminates selection bias.
2. Balances arms with respect to prognostic
variables (known and unknown).
3. Forms basis for statistical tests, a basis for an
assumption-free statistical test of the equality of
treatments. Dr. Wafaa El-Kassed
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Single blind - participants are not aware of
treatment group.
Double blind - both participants andinvestigators unaware
Single blind - participants are not aware of
treatment group.
Double blind - both participants andinvestigators unaware
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As important as randomization.
Eliminates co interventions.C0 intervention means, Unintended effectiveinterventions as:
participants use other therapy or change behavior
Study staff, medical providers, family or friendstreat participants differently.
Eliminates biased measurement of outcome
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Tamper-proof randomization
1. Blinding of participants, study staff, lab
staff, -------------
Adherence to study intervention andprotocol.
Complete follow-up.
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Expensive Time consuming
Sometimes ethically questionable.
Subjects are often a highly selected group(selected for willingness to comply withtreatment regimen, level of health, etc.) andmay not be representative of all people who
might be put on the treatment (i.e.,generalizability may be questionable).
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The major strength of this approach isthe random assignment, which is theonly way to make it likely that the
study groups are comparable inpotential confounding variables thatare either unknown or immeasurable.
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Descriptive
Case report
Case series
Survey
Analytic
Observational
Cross sectional
Case-controlCohort studies
Experimental
Randomized
controlled trials
Strength of evidence for causality between a risk factor and outcomeDr. Wafaa El-Kassed
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Observational
Cross sectionalCase-control
Cohort studies
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In this type of research study, either the entirepopulation or a subset selected from it.
From these individuals, data are collected tohelp answer research questions of interest.
It is called cross-sectional because the
information about X and Y that is gatheredrepresents what is going on at only one point
in time.
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The results of a survey done in a Puerto Ricanpharmaceutical factory indicated an
exceptionally high prevalence of
Gynaecomastiaamong employees. Thisfinding led to the hypothesis that exposure to
Oestrogendust might be the cause; serum
concentrations of oestrogen lent support to
the hypothesis. After improvements in dustcontrol in the factory, the epidemic
disappeared.
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These studies provide a snapshot of the
population at a particular time.
These studies are obviously most useful for
conditions that are:
not rapidlyfatal,
not terribly rare,
and/or not routinely brought to medical
attention (e.g., elevated blood pressure,elevated blood cholesterol, many psychiatric
disorders, diet).
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1. Exposure and outcome status are determined
at the same time.
2. Provide estimates of prevalence of allfactors
measured.
3. Fast/Inexpensive - no waiting!
4. No loss to follow up
5. Associations can be studied
6. Can study entire populations or representative
sample.
7. Greatergeneralizability.Dr. Wafaa El-Kassed
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Susceptible to selectionbias (e.g. selective)
Susceptible to misclassification.
Information on all factors is collected
simultaneously, so it can be difficult to
establish a putative "cause' antedated the
"effect'.
Not good for rare diseases or rare exposures
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Casecontrol studies are studies that
compare cases with a disease to controls
without the disease, looking for differences inantecedent exposures.
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As an example, one could select cases of
young women with venous thromboembolism
and compare them to controls withoutvenous thromboembolism
looking for differences in antecedent oralcontraceptive use.
Several such studies have been performed,
generally demonstrating a strongassociationbetween the use of oral contraceptives and
venous thromboembolism.
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Casecontrol studies can be particularly
useful when one wants to study multiplepossible causes ofa single disease, as one
can use the same cases and controls toexamine any number of exposures as
potential risk factors.
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Using casecontrol studies, one can study rare
diseases with markedly smaller sample sizes
For example, the classic study of
diethylstilbestrol and clear cell vaginaladenocarcinoma required only 8 cases and40 controls, rather than the many thousands
of exposed subjects that would have beenrequired for a cohort study of this question.
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Because subjects are identified after the
disease has developed, the case-controlstudy is sometimes referred to as a
"retrospective" or "backwards" design.
Case-control studies can use prevalent cases
(i.e., existing at the time the study begins) or
incident cases (i.e., newly diagnosed during
the period of the study).
Sample chosen on the basis of outcome
(cases), plus comparison group (controls)Dr. Wafaa El-Kassed
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+ Rare outcome/Longlatent period
+ Inexpensive and efficient: maybe onlyfeasible option
+ Establishes association
+ Usefulfor generating hypotheses (multiple riskfactorscanbe explored)
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1. Causality still difficult to establish
2. Selection bias (appropriate controls)
3. Recall bias: sampling (retrospective)4. Cannot tell about incidence or
prevalence
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A cohort (follow-up, longitudinal) study is acomparative, observational study in whichsubjects are grouped by their exposurestatus,
i.e., whether or not the subject wasexposed to a suspected risk factor
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The subjects, exposed and unexposed tothe risk factor, are followed forward in time
to determine if one or more new outcomes(diseases) occur
Subjects should not have outcomevariable on entry
No new subjects allowed in after initialrecruitment
The rates of disease incidence among theexposed and unexposed groups aredetermined and compared.
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Selection of sample from populationMeasures 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, MinnesotaDr. Wafaa El-Kassed
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Know that predictor variable was presentbefore outcome variable occurred (someevidence of causality)
Directly measure incidence of a diseaseoutcome
Can study multiple outcomes of a singleexposure (RR is measure of association)
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Expensive and inefficient for studying rareoutcomes HERS vs. WHI
Often need long follow-up period or a very largepopulation
CARDIA
Loss to follow-up can affect validity of findings
Framingham
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Retrospective cohort Identification of cohort, measurement of
predictor variables, follow-up and
measurement of outcomes have all occurredin the past
Much less costly than prospective cohorts
Investigator has minimal control over study
design
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Pharmacovigilance involves the detection ofunexpected, and often undesirable, adverseeffects of drugs.
Pharmacoepidemiology, often considered asub-domain in pharmacovigilance, attempts
to quantify the frequency of these adverse
effects, and to identify sub-populations for
which there are variations in the magnitudeof effects.
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