COHORT STUDY

Dr Manish Chandra PrabhakarMGIMS Sewagram

Cohort studieslongitudinal Prospective studiesForward looking study IIncidence studyFollow up study

starts with people free of disease assesses exposure at “baseline”assesses disease status at “follow-up”

Cohort studies

Purpose:- Study of the association between an

exposure factor and one or more diseases outcomes

- Confirmation of hypothesis (a step further towards causation)

• Cohort study is undertaken to support the existence of association between suspected cause and disease

A major limitation of cross-sectional surveys and case-control studies is difficulty in determining if exposure or risk factor preceded the disease or outcome.

Cohort Study:

Key Point:

Presence or absence of risk factor is determined before outcome occurs.

INDICATION OF A COHORT STUDY

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 sample funds are available

Design:

Two groups are studied:- Study group (cohort- Control group

Defined according to the presence or the absence of the exposure under

investigation

Study group (cohort)

Defined as those who:- Have the exposure factor under the study

(working definition of the exposure should be set)

- Are free from the disease or outcome under study and,

- Are at risk of developing this disease or outcome

Study group (cohort)

Selected to represent all the population of exposed individuals

Source of selection may be;-workplace-Registries- Hospital records- community

WHAT IS COHORT

Ancient Roman military unit, A band of warriors.

Persons banded together. Group of persons with a common

statistical characteristic. [Latin]

.

Control group Similar to the study group in everything

except the exposure under the study Must have a similar chance of the

occurrence of the outcome, compared to the study group

Source: general pop., neighbors, friends of cases, hospital…

Matching of factors that might affect the studied relationship (confounding factors)

After selection of the two groups

Baseline information should be obtained from members of the two groups. This information includes:

- Exposure factor: to ensure its presence in the study group and its absence in the control group

- Outcome: to ensure its absence in both groups

- Confounding factors: to assess their presence and level

Follow up A rigorous system of follow up should be

planned and implemented to avoid losses The same follow-up maneuver should be

done in both groups In each follow-up, subjects are assessed

for the occurrence of the outcome under study (endpoint) which must be:

- objectively measurable (hard point) - specific -validAscertainment of exposure status in each

follow-up visit

Follow up

The period of follow-up is determined by the nature of the expected outcome and its latency from the exposure, it should be:

- not too short: no enough time for the outcome to occur

- not too long: probability of the occurrence in the control group

approaches that in the study group

Follow up (historical(

disease

historical follow-up

Control group no disease

Past (1945)

Start

FU

TimeOf

The study

Today 2007

Follow up (concurrent)

Time

Of The stu

dy

Start

FU

Concurrent FU

disease

No disease

Future

2010

In either study

Identification and classification of cohort is based on exposure status

Disease is not present at the start of FU Follow-up is in the direction of the natural

history of the disease

ANALYSIS

Calculation of incidence rates among exposed and non exposed groups

Estimation of risk

Incidence rates of outcome

a+b+c+d

dc

ba

Diseased Not diseased

Yes

No

a+b

c+d

b+d a+c

Total

Control (not exposed)

Study cohort

(exposed)

Incidence rates

Incidence rate among exposed=Ie = a/a+b Incidence among unexposed=Iu = c/c+d

Total incidence= a+c/a+b+c+d

Estimation of risk= measures the strength of association between exposure and outcome

Relative Riskincidence of disease among exposed

RR = ______________________________Incidence of disease among non-

exposeda/a+b

= _________c/c+d

Attributable Risk( measures the absolute increase in the risk of disease due to exposure)

Attributable Risk

Incidence of disease among exposed – incidence of disease among non exposed

AR = _______________________________Incidence of disease among exposed

a/a+b – c/c+dAR = _______________

a/a+b

Smoking Lung cancer Total

YES NO

YES 70 6930 7000

NO 3 2997 3000

73 9927 10000

Find out RR and AR for above data

Incidence of lung cancer among smokers70/7000 = 10 per 1000

Incidence of lung cancer among non-smokers

3/3000 = 1 per thousandRR = 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)

Cohort studies Strengths We can find out

incidence rate and risk

More than one disease related to single exposure

can establish cause - effect

good when exposure is rare

minimizes selection and information bias

Weaknesses losses to follow-up often requires large

sample ineffective for rare

diseases long time to

complete expensive Ethical issues Status change with

long follow up Change in diagnosis

along FU