Comunicación y Gerencia 22/12/20101Cohort studies.

Comunicación y Gerencia

22/12/2010 1Cohort studies

Cohort Studies Cohort Studies

Dr. Salwa Dr. Salwa TayelTayel

Associate ProfessorAssociate ProfessorFamily & Community Medicine DepartmentFamily & Community Medicine Department

King Saud UniversityKing Saud University


Objectives

1. To understand the basic design features of cohort studies

2. To list the advantages and disadvantages of case control studies

3. To calculate measure of association in cohort studies


Types of Study Designs

Design Study Type

Case report Observational - Descriptive

Case series Observational - Descriptive

Cross sectional Observational - Descriptive

Case control Observational - Analytic

Cohort Observational - Analytic

Clinical trial Experimental - Analytic

422/12/2010 Cohort studies

Basic Question in Analytic Epidemiology

Are exposure and disease linked?

Direction of inquiry in cohort studyDirection of inquiry in cohort study

ExposureRisks e.gTobacco chewing

Diseasee.g. Myocardial Infarction (MI)


Analytic Epidemiologic Study DesignsAnalytic Epidemiologic Study Designs

2- Cohort Studies2- Cohort Studies

an “observational” design where the investigator an “observational” design where the investigator

categorizes individuals on the basis of categorizes individuals on the basis of exposureexposure

thus comparing individuals with a known risk thus comparing individuals with a known risk

factor or exposure with others without the risk factor or exposure with others without the risk

factor or exposurefactor or exposure

looking for a difference in the risk (incidence) of a looking for a difference in the risk (incidence) of a

disease over time.disease over time.


What is a cohort?What is a cohort? Cohort is a group having a common characteristic Cohort is a group having a common characteristic

Example: A smoker’s cohort means all are smokers in Example: A smoker’s cohort means all are smokers in

that group that group

A cohort, which is exposed to a suspected factor but A cohort, which is exposed to a suspected factor but

not yet developed the disease, is observed and not yet developed the disease, is observed and

followed over time.followed over time.

Then, the incidence of the disease is measured Then, the incidence of the disease is measured

directly.directly.22/12/2010 7Cohort studies

Key Basis for selection of group for study

= present = absent

Prospective Cohort Study

?

?

Exposure Disease


Coh

ort

Desig

n

timeStudy begins here

Studypopulation

free ofdisease

Exposurepresent

ExposureNot present

disease

no disease

disease

no disease

presentfuture


Cohort Study (Prospective)

Exposed

Unexposed

Disease occurs

No disease

No disease

Disease occurs

Future2010 Present22/12/2010 10Cohort studies

It is the best observational design. Why?It is the best observational design. Why?

The investigator proceeds from “The investigator proceeds from “E to DE to D” i.e. from ” i.e. from

cause to effect so he will not face a chicken egg cause to effect so he will not face a chicken egg

dilemma and the temporal (time) sequence between dilemma and the temporal (time) sequence between

E and D can be clearly established. E and D can be clearly established.

It uses a control group to accept or reject the It uses a control group to accept or reject the

hypothesis between E and D.hypothesis between E and D.


Issues in analysis:Issues in analysis:

The basic analysis involves:

Calculation of incidence rates among

the exposed = (a/a+b)

Calculation of incidence rates among

the non-exposed = (c/c+d )


Frame work of a cohort

OutcomeOutcome

TotalTotalDiseasedDiseased Not Not diseaseddiseased

ExposedExposed

Non-exposedNon-exposedaa

ccbb

dda + ba + b

c + dc + d

(a/a+b)(c/c+d )

Incidence


Quantify risk:Quantify risk:

(Io) exposed non among Incidence

(Ie) exposed among Incidence risk Relative

RR is measure of association between incidence of

disease and certain exposure

Relative risk, Risk Ratio (RR) answers the question:

“How many times a person who is exposed to risk factor

is at risk of developing disease compared to non-

exposed?”22/12/2010 14Cohort studies

a / (a + b)a / (a + b)

c / (c + d)c / (c + d)==Relative RiskRelative Risk

DiseaseDisease

ExposureExposure YesYes NoNo TotalTotal

YesYes aa bb a + ba + b

NoNo cc dd c + dc + d

TotalTotal a + ca + c b + cb + c a + b + c + da + b + c + d


Interpretation of Relative Risk (RR)

RR=1: No associationNo association between exposure and

disease

incidence rates are identical between groups

RR> 1: Positive association (increasedincreased riskrisk)

exposed group has higher incidence than non-

exposed group

RR< 1: Negative association (protective effectprotective effect)

non-exposed group has higher incidence22/12/2010 16Cohort studies

Example:

Incidence in smokers = 84/3000 = 28.0/1000Incidence in smokers = 84/3000 = 28.0/1000Incidence in non-smokers = 87/5000 = 17.4/1000Incidence in non-smokers = 87/5000 = 17.4/1000Relative risk = 28.0/17.4 = 1.61Relative risk = 28.0/17.4 = 1.61


1. Valuable in rare exposures.

2. Can study multiple effects of a single exposure.

3. Exposure happened before outcome (time relation is established) (Temporality)

4. Can calculate incidence rates.

5.5. Can quantify Risk, Relative risk, & Attributable RiskCan quantify Risk, Relative risk, & Attributable Risk

6.6. Dose response ratio can be calculated.Dose response ratio can be calculated.

7.7. Low potential for bias than case-control studyLow potential for bias than case-control study


1.1. Attrition (loss to follow up) may affect validity Attrition (loss to follow up) may affect validity of results.of results.

2.2. Measurement errors, multiple interviews, tests Measurement errors, multiple interviews, tests

3.3. Involve a large sampleInvolve a large sample

4.4. Inefficient for evaluation of rare diseases.Inefficient for evaluation of rare diseases.

5.5. Takes a long time.Takes a long time.

6.6. Expensive.Expensive.


20

Thank YouThank You

Website http://faculty.ksu.edu.sa/73234/default.aspx

[email protected]

22/12/2010 Cohort studies

Comunicación y Gerencia 22/12/20101Cohort studies.

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