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Study Designs in EpidemiologicResearch

Alam Zeb AmiR

Group members: Wajid Shah , Alam Zeb , Faryal

TYPES OF PRIMARY STUDIES

Descriptive studiesdescribe occurrence of outcome

Analytic studiesdescribe association between exposure and outcome

ANALYTIC STUDIES

Exposure Disease

Stu

d y D

esig

ns Case report

Case series

DescriptiveEpidemiology

Descriptive

Experimental Studies

• RCT

• Cross-sectionalstudy

• Case-Crossoverstudy

• Case-Controlstudy

Observational Studies

• Cohort study

Analytic

• Ecologic study

OBSERVATIONAL STUDIES

Case-Control Studiesan “observational” design comparing exposures in disease cases vs healthy controls from same population

exposure data collected retrospectively

CASE-CONTROL STUDIES

Cases: DiseaseControls: No disease

CA

SE-C

ON

TR

OL

DES

IGN

Studypopulation

Cases(disease)

Controls(no disease)

factor present

factor absent

factor present

factor absentpresent

past

time

Study begins here

CASE CONTROL STUDY

Unhygienic Food

Unhygienic Food

Hygienic Food

Hygienic Food

Comparison

PresentPast

Colon

Cancer

NoCancer

CASE-CONTROL STUDY

Strengths

Less expensive and time consumingEfficient for studying rare diseases

Limitationsone cannot directly obtain absolute risk of

an outcome

Exposure measurements taken after disease occurrence

OBSERVATIONAL STUDIES

Cohort Studiesan “observational” design comparing

individuals with a known risk factor or exposure with others without the risk factor or exposure

looking for a difference in the risk (incidence) of a disease over time

data usually collected prospectively (some retrospective)

CO

HO

RT D

ES

IGN

time

Study begins here

Studypopulation

free ofdisease

Factorpresent

Factorabsent

disease

no disease

disease

no disease

presentfuture

COHORT STUDY

Smoker

Non-Smoker

Lung Cancer

Lung Cancer

No Lung Cancer

No Lung Cancer

Comparison

Present Future

COHORT STUDY

StrengthsExposure status determined before disease

detectionSubjects selected before disease detectionCan study several outcomes for each exposure

LimitationsExpensive and time-consumingInefficient for rare diseases or diseases with

long latencyLoss to follow-up

COMPARISON

Case Control CohortRetrospective Prospective/ Longitudinal

Hospital Based Community based

Less time consuming More time consuming

Small sample size Large sample size

Less expensive Very expensive

Rare diseases Common disease

Easy to conduct Difficult to conduct

OBSERVATIONAL STUDIES

Cross-sectional studies An “observational” design that surveys

exposures and disease status at a single point in time (a cross-section of the population)

time

Study only exists at this point in time

CROSS-SECTIONAL DESIGN

time

Study only exists at this point in time

Studypopulation

No Disease

Disease

factor present

factor absent

factor present

factor absent

CROSS-SECTIONAL STUDIES Often used to study conditions that are

relatively frequent with long duration of expression (nonfatal, chronic conditions)

Not suitable for studying rare or highly fatal diseases or a disease with short duration of expression

DisadvantagesWeakest observational design, (it measures prevalence, not incidence of disease).

Usually don’t know when disease occurred

EXPERIMENTAL STUDIES

Randomized Controlled Trials (RCTs)

the “gold standard” of research designs a design with subjects randomly assigned to

“treatment” and “comparison” groupsThe subjects in the study who actually

receive the treatment of interest are called the treatment group.

The subjects in the study who receive no treatment or a different treatment are called the comparison group.

o Both groups are followed up for a specified periodo There is a focus on the control of bias

EX

PER

IMEN

TAL

DES

IGN

timeStudy begins here (baseline point)

Studypopulation

Intervention

Control

outcome

no outcome

outcome

no outcome

baselinefuture

RANDOMIZATION

A SIMPLE RCT DESIGN

Identified sample of subjects

A

B

Treatment group

Control group/Treatment 2Random

allocation

Parallel Design

RCT'S DESIGNS – CROSS OVER DESIGNS

Identified sample of subjects

Treatment 1A

Treatment 2BRandom allocation

B Treatment 2

A Treatment 1

RCT’S DESIGNS- FACTORIAL DESIGNS

Identified sample of subjects

Treatment 1A

Treatment 2B Random allocation

C

D

Treatment 1+2

Control

RANDOMIZED CONTROLLED TRIALS

Disadvantages Patients are allocated to an undesired treatment

option where patient compliance may be low. Can be expensive and time consuming Often have too few patients or too short a follow-

up period Often imperfect randomisation

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