STUDY DESIGNS

PRESENTED BY: Dr. Parth K. Vachhani

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INTRODUCTION

A study design is a specific plan or protocol for conducting the study, which allows the investigator to translate the conceptual hypothesis into an operational one.

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CONTD.

Observational: Studies that do not involve any intervention or experiment.

Experimental: Studies that entail manipulation of the study factor (exposure) and randomization of subjects to treatment (exposure) groups.

HIERARCHY OF STUDY DESIGN

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OBSERVATIONAL STUDIES

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OBSERVATIONAL STUDIES

Non-experimental Observational because there is no

individual intervention Treatment and exposures occur in

a “non-controlled” environment Individuals can be observed

prospectively, retrospectively or currently.

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DESCRIPTIVE STUDIES

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CASE REPORTS

Detailed presentation of a single case or handful of cases

Generally report a new or unique finding e.g. previous undescribed disease e.g. unexpected link between diseases e.g. unexpected new therapeutic

effect e.g. adverse events

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CASE SERIES

Experience of a group of patients with a similar diagnosis

Cases may be identified from a single or multiple sources

Generally report on new/unique condition

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CONTD. Advantages

Useful for hypothesis generation

Informative for very rare disease with few established risk factors

DisadvantagesCannot study cause and effect

relationshipsCannot assess disease

frequency

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Case Report

Case Series

One case of unusualfindings

Multiple cases of findings

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ANALYTICAL STUDIES

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BASIC QUESTION IN ANALYTIC STUDY

Are exposure and disease linked?

Exposure Disease

ANALYTIC STUDY

Group dataEcologic study

Individual dataCross-sectional studyCase-control studyCohort study

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ECOLOGICAL STUDY

An investigation of the distribution of health and its determinants between groups of individuals.

Unit of study is the aggregate data not individual level.

It is usually be conducted as the first step study for research.

The result is difficult to interpret because of confounding and bias.

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CONTD.

AdvantagesCheap, quick and convenient

since it usually come from existing data

Disadvantages Inability to link exposure with

disease in individual (ecological fallacy)

Limit to control effect of other factors

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CROSS-SECTIONAL STUDIES

An “observational” design that surveys exposures and disease status at a single point of time (a cross-section of the population)

time

Study only exists at this point in time

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CONTD.

time

Study only exists at this point in time

Studypopulation

No Disease

Disease

factor present

factor absent

factor present

factor absent

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CONTD.

Often used to study conditions that are relatively frequent with long duration of expression (nonfatal, chronic conditions)

It measures prevalence, not incidence of disease

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

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ADVANTAGES

Gives general description or scope of problem

Useful in health service evaluation and planning

Baseline for prospective study Low-cost

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DISADVANTAGES

No calculation of risk Temporal sequence is unclear Not good for rare diseases Selective recall can lead to bias

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CASE CONTROL STUDY

An “observational” design comparing exposures in disease cases vs. healthy controls from same population.

Exposure data collected retrospectively.

Most feasible design where disease outcomes are rare.

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CA

SE-C

ON

TR

OL

DESIG

N

Studypopulation

Cases(disease)

Controls(no disease)

factor present

factor absent

factor present

factor absent

presentpast

time

Study begins here

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ODDS RATIO

Odds ratio= ad/bc

Suspected Risk Factor

CasesControl

s

Present a b Absent c d

a+c b+d

ADVANTAGES

Cheap, easy and quick studies

Require comparatively few subjects

Multiple exposures can be examined

Rare diseases and diseases with long latency can be studied

Suitable when randomization is unethical

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DISADVANTAGES

Case and control selection troublesome

Subject to bias

Direct estimation of incidence is not possible

If the incidence of exposure is high, it is difficult to show the difference between cases and controls

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COHORT STUDY

An “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.

One of best observational design. Data usually collected

prospectively (some retrospective)

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CO

HO

RT

DES

IGN

time

Study begins here

Studypopulation

free ofdisease

Factorpresent

Factorabsent

disease

no disease

disease

no disease

presentfuture

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DiseaseTotalPrese

ntAbse

ntExpos

ure presen

t

a b a+b

Exposure

absentc d c+d

CONTD.

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ADVANTAGES

Can establish population-based incidence

Accurate relative risk (risk ratio) estimation

Can be used where randomization is not possible

Selection and information biases are decreased

Multiple outcomes can be studied

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DISADVANTAGES

Lengthy and expensive

May require large samples

Not suitable for rare diseases

Not suitable for diseases with long-latency

Unexpected environmental changes may influence the association

Nonresponse, migration and loss-to-follow-up biases

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EXPERIMENTAL STUDY

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EXPERIMENTAL STUDY

Treatment and exposures occur in a “controlled” environment

Planned research designs Clinical trials are the most well

known experimental design.

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CONTD.

Investigator can “control” the exposure akin to laboratory experiments except living populations are the subjects

Generally involves random assignment to groups

The ultimate step in testing causal hypotheses

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RANDOMIZED CONTROLLED TRIAL

A design with subjects randomly assigned to “treatment” and “comparison” groups.

Provides most convincing evidence of relationship between exposure and effect.

Not possible to use RCTs to test effects of exposures that are expected to be harmful for ethical reasons.

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EX

PER

IMEN

TAL

DESIG

N

timeStudy begins here (baseline point)

Studypopulation

Intervention

Control

outcome

no outcome

outcome

no outcome

baselinefuture

RANDOMIZATION

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The “gold standard” of research designs

Provides most convincing evidence of relationship between exposure and effect

Trials of hormone replacement therapy in menopausal women found no protection for heart disease, contradicting findings of prior observational studies

CONTD.

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Ability to randomize subjects Temporal sequence of cause and

effect Can control extraneous variables Best evidence of causality

ADVANTAGES

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DISADVANTAGES

Expensive It may be unethical to assign

persons to certain treatment or comparison groups

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Other Types of Experimental StudyField trialsCommunity trialsAnimal studies

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META ANALYSIS

Meta-analysis is a statistical analysis of a collection of studies.

Meta-analysis methods focus on contrasting and comparing results from different studies in anticipation of identifying consistent patterns and sources of disagreements among these results.

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THANK YOU