2,3 types of study designs

8/2/2019 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.

Dr. Wafaa El-Kassed


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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

Dr. Wafaa El-Kassed


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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

Dr. Wafaa El-Kassed


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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.

Dr. Wafaa El-Kassed


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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.

Dr. Wafaa El-Kassed


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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.

Dr. Wafaa El-Kassed


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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.

Dr. Wafaa El-Kassed


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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.

Dr. Wafaa El-Kassed


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Descriptive

Case report

Case series

Survey

Analytic

Observational

Cross sectional


Experimental

Randomized

controlled trials



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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

Dr. Wafaa El-Kassed


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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

Dr. Wafaa El-Kassed


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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

Dr. Wafaa El-Kassed


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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

Dr. Wafaa El-Kassed


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PopulationPopulation

SampleSample

Treatment

Placebo

RandomizationRandomizationRandomizationRandomization

Dr. Wafaa El-Kassed


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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.

Dr. Wafaa El-Kassed


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Dr. Wafaa El-Kassed


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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

Dr. Wafaa El-Kassed


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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

Dr. Wafaa El-Kassed


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Tamper-proof randomization

1. Blinding of participants, study staff, lab

staff, -------------

Adherence to study intervention andprotocol.

Complete follow-up.

Dr. Wafaa El-Kassed


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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).

Dr. Wafaa El-Kassed


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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.

Dr. Wafaa El-Kassed


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Descriptive

Case report

Case series

Survey

Analytic

Observational

Cross sectional


Experimental

Randomized

controlled trials



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Observational

Cross sectionalCase-control

Cohort studies

Dr. Wafaa El-Kassed


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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.

Dr. Wafaa El-Kassed


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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.

Dr. Wafaa El-Kassed


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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).

Dr. Wafaa El-Kassed


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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

Dr. Wafaa El-Kassed


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Casecontrol studies are studies that

compare cases with a disease to controls

without the disease, looking for differences inantecedent exposures.

Dr. Wafaa El-Kassed


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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.

Dr. Wafaa El-Kassed


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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.

Dr. Wafaa El-Kassed


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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.

Dr. Wafaa El-Kassed


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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)

Dr. Wafaa El-Kassed


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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

Dr. Wafaa El-Kassed


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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

Dr. Wafaa El-Kassed


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Dr. Wafaa El-Kassed


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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.

Dr. Wafaa El-Kassed


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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)

Dr. Wafaa El-Kassed


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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

Dr. Wafaa El-Kassed


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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

Dr. Wafaa El-Kassed


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Dr. Wafaa El-Kassed


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Dr. Wafaa El-Kassed


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Dr. Wafaa El-Kassed


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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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2,3 types of study designs

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