Present: Disease Past: Exposure

CHP400:Community Health Program - lI

Research Methodology

STUDY DESIGNSObservational / Analytical Studies

CohortStudy Present:

Disease Past: Exposure

Cross - section Cohort

Case - control

Present:Disease & Exposure

Present: Exposure Future: Disease

nbmmb

Time is Key

Content

Definition Characteristics and types Design Advantages and disadvantages Follow-up Analysis

STUDY DESIGNS

ExperimentalObservational

Animal

Experiment

HumanInterventio

nClinical trial

Analytical

Case control

Descriptive

Case report

Case series

Cross section

Ecological

Cohort


CohortStudy

A cohort is a group of people who share a common characteristic or experience

Cohort study: An observational, analytic, longitudinal, epidemiologic study in which a particular outcome, such as death from a heart attack, is compared in groups of people who are alike in most ways but differ by a certain characteristic, such as smoking ( EXPOSURE )

STUDY DESIGNS


Observational / Analytical StudiesCohort Study

Definition:

Characteristics : A “cohort” is a group of people, referred to as

“disease-free population” or “population at risk”

A survey is first carried out to exclude prevalent cases from the cohort

We know the exposure status, looking for the disease status

A period of "follow-up“ is specified, for possible new cases' occurrence

STUDY DESIGNS


Observational / Analytical Studies

Cohort Study

Types:

Two types are recognized: Prospective (longitudinal): forward

in time follow-up study

Retrospective (historical): backward in time study (depends on records: medical / employment). This is the type preferred under occupational settings.

STUDY DESIGNS



Cohort Study

Population

People without disease

Exposed

Not expose

d

DiseaseNo

disease

Disease

No disease

STUDY DESIGNS



Cohort Study

Design :

YES

NO ?

?

TIMEOUTCOME/DISEASEEXPOSURE

COHORT STUDIESCLINICAL TRIALS

STUDY DESIGNS



Cohort Study

YES

NO


YES

NO ?

?

CASE-CONTROL STUDIES ?

?

COHORT STUDIESCLINICAL TRIALS


STUDY DESIGNS



Cohort Study

INDICATIONS: ??

When there is good evidence of exposure and disease.

When exposure is rare but incidence of disease is higher among exposed

When follow-up is easy, cohort is stable When ample funds are available

STUDY DESIGNS



Cohort Study

Elements of cohort study Selection of study subjects Obtaining data on exposure Selection of comparison group Follow up Analysis

STUDY DESIGNS



Cohort Study

Selection of study subjects General population

Whole population in an areaA representative sample

Special Exposure groups of populationOccupation/professional groups

• e.g. Radiologists exposure to X-ray

STUDY DESIGNS



Cohort Study

Personal interviews / mailed questionnaire

Reviews of records

Dose of drug, radiation, type of surgery etc

Medical examination or special test

Blood pressure, serum cholesterol

Obtaining data on exposure

STUDY DESIGNS



Cohort Study

Environmental survey

By obtaining the data of exposure we can classify cohorts as

Exposed and non exposed and

By degree exposure we can sub classify cohorts

Obtaining data on exposure

STUDY DESIGNS



Cohort Study

Internal comparisonOnly one cohort involved in studySub classified and internal comparison done

External comparisonMore than one cohort in the study for the

purpose of comparisone.g. Cohort of radiologist compared with

ophthalmologists

Selection of comparison group

STUDY DESIGNS



Cohort Study

Comparison with general population rates

If no comparison group is available we can compare the rates of study cohort with general population.

Cancer rate of uranium miners with cancer in general population

Selection of comparison group

STUDY DESIGNS



Cohort Study

To obtain data about outcome to be determined (morbidity or death)Mailed questionnaire, telephone calls,

personal interviewsPeriodic medical examinationReviewing recordsSurveillance of death records

Follow-up

STUDY DESIGNS



Cohort Study

Some loss to follow up is inevitable due to death, change of address, migration, change of occupation.

Follow up is the most critical part of the study

Follow-up

STUDY DESIGNS



Cohort Study

Follow-up Techniques :1.Periodical medical examinations and

mailed questionnaires.

2.Direct personal interviews or examinations.

3.Videoconference, neighbors, friends and relatives.

STUDY DESIGNS



Cohort Study

Follow-up Techniques :4. Lost persons can be traced through letters,

their relatives or friends.

5. Migrated cohort subjects can also be traced through travel and immigration

authorities.

6. Dead persons: local or regional Mortality Registers or Death Certificates.

STUDY DESIGNS



Cohort Study

Problems during Follow-up :

Follow-up of a large group. Limited resources. Time constraint. Paucity of trained personnel Attrition

STUDY DESIGNS



Cohort Study

Attrition Reduction : Obtaining an informed consent. Recording commitment to continue and

cooperate in the study. Tracing LOST subjects. Considering Information of lost subjects at

the time of analysis Keeping non-response at a low level to

improve the validity.

STUDY DESIGNS



Cohort Study

Calculation of incidence rates among exposed and non exposed groups

Estimation of risk

Analysis

STUDY DESIGNS



Cohort Study

dc

ba

Yes NoDisease Status

Yes

No

Exposure Status

a+b

b+d a+c

TotalStudy cohortComparison cohort c+d

NTotal

Incidence among exposed =a

a+bIncidence rate Incidence among unexposed =

cc+d

ANALYSIS

STUDY DESIGNS



Cohort Study

Estimation of riskANALYSIS

Relative Risk (RR)

incidence of disease among exposed

Incidence of disease among non-exposedRR = _____________________________

a/a+bc/c+d

_________RR =

STUDY DESIGNS



Cohort Study

Estimation of riskANALYSIS

Incidence of disease among

exposed

Incidence of disease among non exposed

a/a+b – c/c+d

a/a+b

AR

AR

=

=

Incidence of disease among exposed

_______________________________

_______________

Attributable Risk (AR):

STUDY DESIGNS



Cohort Study

Smoking Lung cancer Total

Smoking YES NO Total

YES 70 6930 7000

NO 3 2997 3000

Total 73 9927 10000

Calculate RR and AR for above data

STUDY DESIGNS



Cohort Study

Incidence of lung cancer among smokers

70/7000 = 10 per 1000 Incidence of lung cancer among non-smokers

3/3000 = 1 per 1000

RR = 10 / 1 = 10

(lung cancer is 10 times more common among smokers than non smokers)

AR = 10 – 1 / 10 X 100

= 90 %

(90% of the cases of lung cancer among smokers are attributed to their habit of smoking)

STUDY DESIGNS



Cohort Study

The Ideal Cohort :

An ideal cohort should be: Stable. Cooperative. Committed Well-informed

STUDY DESIGNS



Cohort Study

Advantages: No temporal ambiguity (suggests cause-

effect relationship) Calculation of incidence rates Suitable for rare exposures Several outcomes can be studied, after

follow-up starts. Factors associated with selection cannot

influence disease status and hence the results.

STUDY DESIGNS



Cohort Study

Disadvantages :

Expensive Time-consuming May be impractical Loss to follow-up may affect sample-

size

STUDY DESIGNS



Cohort Study

Documents

Present: Disease Past: Exposure