Epidemiological Study Methods

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EPIDEMIOLOGICAL

STUDY METHODS

OKETADE SOA


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OUTLINE

INTRODUCTION

DEFINITIONS CLASSIFICATION

STUDY DESIGNS

VARIOUS DESIGNS CONCLUSION


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INTRODUCTION

The science of epidemiology has matured

significantly from the times of Hippocratesand john snow [physician] that the

techniques for analysing data vary depending

on the type of dxs being monitored but eachstudy will have similarities. environmental

factors can influence the occurrence of the

diseases.

epidemiology study of what is upon the people

Derived from the greek terms epi=upon,among.

demos=people, district. Logos study, word.


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DEFINITION OF EPIDEMIOLOGY

Epidemiology is the study of distribution and

determinant of health related state or event in

a specified human population and the

application of this study to the control of healthproblem.


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EPIDEMIOLOGY [DEFINITION OF KEY TERMS]

Distribution : Frequency (including rates &

risks) & pattern of health events(person,

place, time)

Determinants : factors or events that are

capable of bringing about a change in health

Human population : Epidemiology examines

health events among population groups rather

than individuals.


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EPIDEMIOLOGY [DEFINITION OF KEY TERMS]

Health related states: infections, chronic

diseases & physiological events &various

states of health such as disability, injury,

mortality

Health related events : immunization, hospital

attendance, bed occupancy

Application : basis for directing interventions


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CLASSIFICATION


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CLASSIFICATION(CONT.)

CORRELATIONAL


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OBSERVATIONAL VS EXPERIMENTAL STUDIES

Observational studies

Allow nature to take its cause; the investigator

measures but does not intervene

Descriptive study: focuses on the description

of the occurrence of a disease in a population

Analytical study analyses relationshipsbetween health status and other variables


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OBSERVATIONAL VS EXPERIMENTAL STUDIES

Experimental or interventional studies: involve

an active attempt to change a disease

determinant(e.g an exposure or a behaviour)

or the progress of a disaese (through

treatment)

The studies are based on a grp which has had

the experience compared with control grpwhich has not had the experience.


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PURPOSE OF DESCRIPTIVE

EPIDEMIOLOGY

To generate hypothesis

To permit evaluation of trends in health &

disease and comparisons among countries

and subgroups within countries.

To provide a basis for planning, provision and

evaluation of health services

To identify problems to be studied by

analytical methods and to suggest areas that

may be fruitful for investigation


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CASE STUDIES(CASE SERIES)

Case reports:documents unusual medicaloccurrence and can represent the first clues tothe formulation of hypothesis, generallyreport a new or unique findings and previousundescribed disease.

Case series: collection of individual case

reports which may occur within a fairly shorttime, and experience of a group of patientswith similar diagnosis.


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Case Series

AdvantagesUseful for hypothesis generation

Informative for very rare disease with few established

risk factorsUsually of short duration.

Disadvantages

Cannot study cause and effect relationships

Cannot assess disease frequency

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CROSS-SECTIONAL STUDY

It is also called epidemiologic study or prevalence study

It analyses (describes)data collected on a group ofsubjects at one point in time rather than over a period

of time. i.e they survey exposure and disease at asingle point in time.

Both exposure and outcome variables are beenevaluated at the same point in time(without any inbuiltdirectionality)

Most sophisticated descriptive study

It answers the question WHAT IS HAPPENING RIGHTNOW?


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QUESTION: WHAT IS HAPPENING?NO DIRECTION OF INQUIRY

o onset time end

subjects

With

outcomeWithout

outcome


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CROSS-SECTIONAL STUDY

ADV

Best for determining

the status

quo(prevalence) Quick

Relatively inexpensive

DISADV

Only a snapshot at a

time leading to a

misinformation Response rate may be

low ,with result not

representative of the

population


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Cross-sectional studies

DisadvantagesWeakest observational design,

(it measures prevalence, not incidence of

disease). Prevalent cases are survivorsThe temporal sequence of exposure and effect

may be difficult or impossible to determine

Usually dont know when disease occurredRare events a problem. Quickly emerging

diseases a problem

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CORRELATIONAL STUDY DESIGN

A study comparing incidence/prevalence of

one event against another on a global scale

Measures that represent characteristics of

entire populations are used to describe the

disease in relation to some factor of interest

(such as age, calendar time, food

consumption, drug use and utilization ofhealth services)


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CORRELATIONAL STUDY DESIGN

ADV

Compares events amongnations

DISADV

Doesnt compare

individuals, so it might leadto overgeneralization.


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ANALYTICAL STUDIES

Two basic designs:

Case control or retrospective study

Cohort or prospective

NOTE

There must be a comparison group No control No conclusion(NCNC)


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CASE CONTROL OR CASE HISTORY

STUDY A group of affected people is compared to unaffected

people(the control)

Its a LONGITUDNAL STUDY (like cohort study) because itsa study over a period of time.

Subjects are selected based on a particular outcome and astudy backwards in time to try to detect the causes or riskfactors that may have earlier been reported in a descriptivestudy

Subjects are then matched and assigned into the two

groups. Subject selected on the basis of disease[e.g lungcancer].

Sometimes called a retrospective study because of thedirection of study


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CASE CONTROL OR CASE HISTORY

STUDY


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Advantages of case control

It is relatively easy to carry out bcos we go

back to existing records in the hospital

It is also rapid and inexpensive

It requires comparatively few subjects

It can assist one in studying different

etiological factors

One does not need an ethical clearance

There is no risk to the subject


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Disadvantages of case control

It introduces bias

To select an appropriate control could be

difficult

It may be difficult to distinguish between the

cause of a disease and an associated factor


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COHORT STUDY

A cohort is a grp of people who have somethingin common and remain part of a group over anextended time

A group of people exposed to a suspectedetiological agent are compared with a matchedcontrol who have not been similarly exposed.Subject selected on the basis of exposure

[aetiological factor; cigarette smoking] Follow-up over a period to compare the outcome

Also a longitudinal study or prospective study


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ADVANTAGES OF COHORT

There is no bias

The risk can be calculated bcos the incidence

can be calculated

It is effective for studying rare exposures

It allows the study of the natural history of the

disease

It assists in determining the temporal

relationship between the etiological factor &

the disease


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Disadv of cohort study

It takes a long time

It is expensive

Large no of subjects are needed There could be changes in the standard

methods or diagnostic criteria


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EXPERIMENTAL STUDIES

Studies in which 1 grp is deliberately

subjected to an experience compared with a

control group with no similar experience

The gold standard in medicine bcos it proves

causality

Can be controlled or uncontrolled


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UNCONTROLLED EXPERIMENTAL

STUDIES

Intervention is not compared with a control

The aim is to confirm that the Intervention

made a difference


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CONTROLLED EXPERIMENTAL STUDIES

In this study, a drug or procedure is compared

to:

1. Another drug

2. Procedure

3. Placebo

4. Previously accepted tx The aim is to proove the difference due to tx


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CONTROLLED EXPERIMENTAL STUDIES

Blind trial-single or double

Control could be:

A. METHODOLOGY

1. Concurrent or parallel: randomized or non-

randomized(quasi)

2. Sequential control: self controlled or crossover

3. External control


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B. STUDY POPULATION

1. Clinical trials

2. Field trials

3. Community trials


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EXPERIMENTAL STUDIES

ADV

Best study type

Greatest proove of causality

Gold standard for otherdesign

Least bias

Proves best tx or procedure

efficacy

DISADV

Greatest expense

Long duration

Unproven facts adopted bycommunity can hinder

study acceptance


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overview


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CONCLUSION

What you cant measure you cant

control!epidemiological study methods are

used to study your health and my health and

its determinants, as we join hands to ensure ahealthier us.


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Epidemiological Study Methods