Pemilihan Disain Studi Observasional

8/14/2019 Pemilihan Disain Studi Observasional

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Research Design:Choosing the Study Design

Kimberly R. BarberDirector of Research

McLaren Regional

Medical Center


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Yes No

Yes No Yes No

Exposure Outcome Exposure & OutcomeSimulteneous

Exposure Outcome

Assign Exposure?

Experimental Observational

Random allocation Comparison group

RCT Non-RCT

Analyticstudy

Descripstudy

Direction?

Cohort Case-

Control

Cross-

Section


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Design to Employ

Dependent on the hypothesis posed.

Is your intent to observe, associate factors, orshow cause and effect?

Are exposure or outcome factors common or

rare?

Are your resources many or constrained?


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Types of Study Designs

Observational Designs

Descriptive Analytic

Cross-sectional, Case-control, Cohort

Experimental Designs

Quasi experimentalNon randomized or non control trial

True experimentalRandomized Controlled Trial


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Descriptive Studies

No assignment of exposure or risk factor.

Intent is to observe and record.

Record events or activities. Single event or case = Case Report.

Several events or cases = Case Series.


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Cross-Sectional Studies

Measurement of risk and outcome at the

same time.

Outcome

Risk factor


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Cross-Sectional Design

The only study capable

of calculating

prevalence. Proportion of the

population with

the outcome at any

point in time.


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Cross-Sectional Studies (cont)

Most useful if exposure continues right up to

time that outcome is recognized.

Often conducted as screening and

classification preamble to prospective studies.


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Application Exercise:

Cross-Sectional Study

Design a cross-sectional study that examines

the relationship between dietary sodium andhypertension in middle-aged men.

What research question can we answer?


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Advantages

Cheap and quick studies.Data is frequently available through current

records or statistics.

Ideal for generating new hypothesis.


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Disadvantages

The importance of the relationship between the

cause and the effect cannot be determined. Temporal weakness:

Cannot determine if causeprecededthe effect or

the effect was responsible for the cause.

The rules ofcontributory cause cannot be fulfilled.


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Case-Control Studies

(Retrospective)

Case/Control Studies.

Group subjects according to the presence orabsence of the outcome.

Review past histories of the subjects for the

occurrence of suspected risk factors.

?

?

DiseaseRisk


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Case / Control Studies (cont)

Case / Control studies have two main

purposes:Descriptive

Describe the risk factorprofile for an outcome.

Analytic Analyze associations between outcome and risk

factors.


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Application Exercise:

Case / Control Study

Describe a case/control study on the

relationship between childhood obesity,smoking history, and occurrence of

hypertension in middle-aged men.

What research question can we answer?


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Case / Control Studies

Advantages

Are good initial explanatory studies.

Investigators can explore multiple risk factorssimultaneously for one outcome.

Are efficient, relatively cheap, and quick. Data available through

chart review.


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Case / Control Studies Advantages

(cont)

Well suited for rare diseases.

Since the study begins with subjects who alreadyhave the outcome, it is easier to accumulate

enough subjects for significant results.


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Case / Control Studies

Disadvantages

Data Quality

Data with inadequate detail, questionablereliability, or use a different standard to judge

disease severity.

Recall bias

Subjects who have unpleasant experiences mayrecall past differently than control subjects.


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Retrospective Studies

Disadvantages (cont)

Sampling bias

Sample usually not representative of all subjectswho could be included.

Other Capable of studying only one outcome at a time

Cannot calculateprevalence or incidence. Subject to confounding factors.

Cannot prove contributory cause.


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Cohort Studies(Prospective)

Subjects identified according to the presence

or absence of the risk factor.

Followed over time until the outcome occurs

or becomes evident.


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Cohort Studies (cont) Subjects with and without the suspected risk

factorare followed for the development of the

outcome.

The frequency of the outcome is comparedbetween the two groups.

Disease

?

?

R

i

s

k


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Cohort Studies (cont)

Cohort studies have two main purposes:

Descriptive Describe the incidence of outcome over time.

Analytic Analyze associations between risk factors and

outcome.


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Historical Cohort Studies

(Non concurrent)

The following all happen in the past:

Assembly of the cohort according to the presenceof absence ofrisk factors.

Baseline measurements

Follow-up and outcomes

Exposure

?

?

Disease


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Historical Cohort (cont)

Are not retrospective studies.

Subjects are grouped according to the presenceor absence ofrisk factors (past or present).

Subjects in retrospective studies are grouped

according to the presence or absence of theoutcome.


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Cohort Studies

Advantages

More powerful design for defining incidence.

Powerful design forassociating the causewith the effect. Can suggest that the cause precedes the effect.

Data can be collected in a comprehensible and

uniform fashion.Exception: historical cohort with same measurement

weaknesses as the case/control.


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Cohort Studies

Advantages (cont)

No recall bias.

Cohort designs can examine many outcomesfor potential risk factors under investigation.


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Cohort Studies

Disadvantages

Expensive in time, money, and number of

patients necessary to demonstrate significant

differences between groups.

Less likely than retrospective studies to

uncover new risk factors.

Also subject to confounding.


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Cohort Studies

Disadvantages (cont)

Loss of valuable information due to patient

attrition.

Patients may change their behaviors or risk

factors after the initial grouping of subjects

resulting in misclassification and studyerror.


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Clinical Trial

Experimental study

Unique features: Intervention in the subjects care for the

purposes of the study.

Randomization of subjects.

Control group comparison.Placebo or treatment


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Clinical Trial Design

Randomized

Intervention/

Effect

Outcome

X

X


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Clinical Trial (cont)

Randomization

Subjects are randomly assigned to control or experimental

group.

Groups are similar in every way except for the intervention

under study.

Each subject has equal probability of being placed in

either group.


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Clinical Trials

Advantages

Subject to the fewest methodological biases

of all study designs.

Most powerful study designed to show

contributory cause.


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Clinical Trials

Disadvantages

Is the most expensive study design in terms

of money, time, and number of patients. Issues of patient attrition and compliance may

invalidate the results.

Can be problematic forethical reasons.

Use of placeboHarm outweighing benefits

Zero tolerance for some exposures


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Choosing the Specific Design

Study design is highly dependent on the type

of analysis. (3)

Type of analysis is dependent on the

hypothesis posed. (2)

The hypothesis is dependent on the intent of

your research. (1)


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1. Research Study Intent

Know the problem.

Determine what you

want to conclude.

Formulate the question.


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Examples of Intent

I intend to show that aspirin resistance isassociated with the severity of heart disease. I will compare levels of aspirin resistance among

patients with differing severity of heart disease.

I intend to show that breast feeding isprotective against allergies developing in

infants. I will compare rates of allergies among women

who breast feed and those who do not.


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2. Research Hypothesis Know the question you

want answered.

Restate the question into

terms of Hoand H

a.

Think aboutcorresponding analysis.


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Examples of Hypotheses

Is aspirin resistance associated with heart

disease? Aspirin resistance increases the risk of heart

attack.

Is breast feeding associated with allergies?

Breast feeding decreases the risk of allergies inbabies.


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3. Statistical Plan of Analysis

Correlation?

Comparison?

Association?

Difference?


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Examples of Analysis

The level of aspirin resistance is compared

between those with heart attack and those

without. Differences in resistance scores between two

groups (Student T-test)

The rate of infant allergies is comparedamong women who breast feed and those

who do not. Relative risk association (Chi square analysis)


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Choose Appropriate Design

Cross-sectional.

Case-control.

Cohort.

Clinical Trial.


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Apply the Best Design

Think about the

measures to be used.

Know the analysis

required.

Rethink desired

conclusions.


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Summary

Study Design key to determining cause or just

association. Each design has unique strengths &

weaknesses.

The design drives the conclusions.

Pemilihan Disain Studi Observasional

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