5. cohort studies

Cohort Study

Dr Naveen PhuyalMBBS,MD

Synonyms

• Longitudinal study• Prospective study• Incidence study• Forward-looking study

What is a Cohort?

• Group of people who share a common characteristic or experience within a defined time period.

AgeOccupationExposure to drug or vaccinePregnancyInsured person

• An ancient Roman military unit, comprising six centuries, equal to one tenth of a legion.

• A group of people banded together or treated as a group.

What is a Cohort?

When is a cohort study warranted?

• Good evidence of association between exposure and outcome as shown by descriptive and case control studies

• When exposure is rare but incidence is high among exposed

• When attrition of people can be minimized

• When ample of funds are available

Research methods

Observational

Descriptive

Case series, case reports,

CS, cohort

Analytical

Ecological Cross-sectional

Case control Cohort

Experimental

ControlledUncontrolled,Non-random

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Position in the evidence hierarchy

Design of a cohort study

Cohort study

Features of Cohort study


• Cohorts are identified prior to appearance of disease under investigation

• Study groups are observed over a period of time to determine the frequency of disease among them

• Study proceeds forward from cause to effect

• Key element of Cohort study is time.

• Following the exposed and unexposed groups over time cohort studies are uniquely equipped to describe process and mechanisms by which exposures relate to the development of disease.

• Cohort study is the primary tool to study time and medicine ( Samet 2000)


• They provide the data to describe when disease occur and track their consequences over time.

• Factors that cause disease or early signs of disease can be monitored over time.

• Diversity of individuals in cohort study: provides data to identify risk factors that make certain individual more susceptible.


• Data collected in cohort studies are useful to describe the prognostic markers of exposure.

• Multisite cohort studies may serve additional role of characterizing where disease occur and to what extent the diseases are spread in different locations.

• A cohort study with adequate sample and follow up enables us to understand natural history of disease.


• Once a measure of frequency of disease occurrence (incident cases in person-years) is adopted, cohort studies allow the direct comparison of the risk of becoming ill in several groups.

• This comparison can be relative or absolute– Relative: how many times higher or lower is the risk between

exposed and unexposed (relative risk)

– Absolute: how much difference in risk is there between exposed and unexposed ( attributable risk)


• Relative risk will give you causal relationship between disease and exposure.

• Attributable risk measures the change of incidence due to exposure in question ( it quantifies the burden of disease that an exposure exerts in a population.)

• Identification of exposures and risk factors for a disease forms the basis for prevention.


Cohort based associations that have resulted in prevention strategies

• Lung Ca and smoking: avoid cigarette• Unprotected sex and HIV: avoid• IV drug use and HIV: avoid• To prevent heart disease: avoid high LDL and

low HDL levels• To prevent Cervical cancer: avoid Papilloma

infection

General consideration

• Cohorts must be free from disease under study.

• Both the study and control groups should be equally susceptible to the disease under study

• Both groups should be comparable in respect to all possible variables

• Diagnostic and eligibility criteria of disease must be defined beforehand.

Types of cohort studies

Types of cohort studies

1. Prospective cohort studies

2. Retrospective cohort studies

3. Combination of retrospective and prospective cohort studies

Prospective cohort study

Retrospective cohort study

Retrospective plus prospective cohort study

Elements of cohort study


• Selection of study subjects• Obtaining data on exposure• Selection of comparison groups• Follow up• Analysis



1. Selection of study subjects1. General population

when exposure (cause) of death is frequent in general population, residing in well defined geographical, political and administrative areas.Eg. Framingham heart study

2. Special groupsSelect groups: doctors, nurses, lawyers, teacherscollege alumni, employees, volunteers.

Exposure groups: special exposure to physical, chemical and other agents.

1. Selection of study subjects



2. Obtaining data in exposure

• Cohort members: interviews/ questionnaires

• Review of records: dose of radiation/ kinds of surgery/ details of medical treatment

• Medical examination or special tests: blood pressure, serum cholesterol, ECG

• Environmental surveys: where the cohort lived or worked

• Information about exposure will allow classification of cohort members:

1. According to whether or not they have exposed to suspected factor

2. According to degree of exposure at least in broad class in case of special exposure groups

2. Obtaining data in exposure



3. Selection of comparison groups

i. Internal comparisons

ii. External comparisons

iii. Comparison with general population rates

Internal comparison

• No outside comparison group required.

• A single cohort enters the study and on basis of information obtained, classified into several comparison groups according to degree or levels of exposure.

External comparison

• When information on degree of exposure is not available

• Eg. Smokers and non-smokers• Cohort of radiologists with cohort of opthalmologists• The study and control cohorts should be similar in

demographic and possibly important variables other than those under study.

Comparison with general population rates

• Frequency of lung cancer among uranium mine workers vs frequency of lung cancer in general population

• Asbestos workers vs general population cancers



4. Follow up

• Periodic medical examinations • Review of records: physician/ hospital• Routine surveillance of death records• Mailed questionnaires, phone calls, periodic home

visits- preferably all three every year

• Loss to follow up occurs due to death, change of residence, migration, withdrawal of occupation

• Achieve 95% follow up as much as possible



5. Analysis1. Incidence rates

1. Among exposed2. Among not exposed

2. Estimation of risk

3. Relative risk

4. Attributable risk

5. Population attributable risk

Disease

Exposure Yes No Total

Yes a b a+b

No c d c+d

Total a+c b+d a+b+c+d

Incidence rates among exposed= (a/a+b)*1000

Incidence rates among non-exposed= (c/c+d)*1000

Relative riskDisease

Exposure Yes No Total

Yes a b a+b

No c d c+d

Total a+c b+d a+b+c+d

Relative risk is the ratio of incidence among exposed to incidence among non-exposed

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

Relative risk

• Direct measure of strength of association between suspected cause and effect

• RR=1 (no association)

• RR>1 (positive association)

• If RR=2 ( incidence rate of disease is 2 times higher among exposed than non exposed or 100% increase in risk)

Cigarette smoking

Lung Ca developed

No Lung Ca devloped

Total

Yes 70 6930 7000

No 3 2997 3000

RR= (70/7000)/ (3/3000)= 10Smokers are 10 times at greater risk Of devloping lung Ca than non smokers

Attributable risk

• Also called as ‘ risk difference’• AR is the difference in incidence rates of disease

(or death) between an exposed group and non-exposed group.

• AR= Incidence of disease rate among exposed – incidence of disease rate among non-exposed

Attributable risk percentageAR%= Incidence of disease rate among exposed

– incidence of disease rate among non-exposed

incidence of disease among exposed

In above example AR= (10-1) X 100 = 90%10

90 percent of lung cancer among smokers was due to smoking.This is the amount of disease that might be eliminated if the factor under study is controlled or eliminated.

X 100

Population attributable risk

PAR= It – INE

PAR %= It – INE

It

PAR % provides the estimate of the amount by which the disease could be reduced in that population if the suspected factor was eliminated or modified.

X 100

RR vs AR

• The size of RR is a better index than AR for assessing the etiological role of a factor in disease

• Larger the RR, stronger the association between cause and effect

• AR gives better idea on impact of successful preventive or public health programme might have in reducing the problem

Advantages of cohort studies

• Incidence can be calculated.• Several outcomes can be studied simultaneously.• Provide direct estimate of relative risk.• Dose response ratios can be calculated.• Certain bias can be minimised.

Disadvantages of cohort studies• Involves large no. of people and unsuitable for study of

diseases with low incidence.• Takes long time to conduct the study.• Loss of staff, loss of fund.• Requires extensive record keeping.• Loss to follow up.• Selection of comparison group is difficult.• Expensive• Study may alter the participants behavior.• Ethical problems.

Case control

• Effect to cause• Starts with disease• Tests weather suspected

cause is more frequent among diseased or undiseased.

• Fewer subjects• Quick results• Suitable for rare disease• OR• Relatively cheap

Cohort

• Cause to effect• Starts with exposure• Tests weather suspected

disease occurs more frequenty among exposed or not exposed

• Large no. of subjects• Long follow up• Suitable for rare exposures• RR/ AR• Expensive

References

• Epidemiology, Fifth edition. Leon Gordis• Park.s textbook of Prevntive and social

medicine, 23rd edition. K. Park• Maxcy-Roseneu-Last, Public Health and

Preventive medicine, 15th edition. Robert B Wallace.

• Oxford textbook of Public Health,Fifth edition

Thank You

Healthcare

5. cohort studies