Cohort Study

COHORT STUDY DR. Mirella Youssef Lecturer of Public Health Community Medicine Department

Cohort studies longitudinal Prospective

studies Forward looking study I Incidence study Follow up study  starts

with people free of disease  assesses exposure at “baseline”  assesses disease status at “followup”

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 -valid Ascertainment 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( Past (1945) Start FU

Today 2007

disease

historical follow-up

Control group no disease

Time Of The study

Follow up (concurrent) Time Of The stud y

Futur e

disease Start FU

Concurrent FU No disease

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

Study cohort (exposed) Control (not exposed)

Diseased

Not diseased

Yes

a

b

a+b

No

c

d

c+d

a+c b+d

Total

a+b+ c+d

Incidence rates  Incidence

Ie

rate among exposed=

= 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

Risk incidence of disease among exposed RR = ______________________________ Incidence of disease among nonexposed a/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+d AR = _______________ 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 smokers 70/7000 = 10 per 1000  Incidence of lung cancer among nonsmokers 3/3000 = 1 per thousand 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)

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