1. Cohort Study
Hari Prasad Kafle
Assistant Professor
Pokhara University

2. Cohort Study

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.

3. Cohort Study

Cohort Study:
Key Point:

Presence or absence of risk factor is
determined before outcome occurs.

4. What is Cohort ?




Ancient Roman military
unit, A band of warriors.
Persons banded together.
Group of persons with a
common statistical
characteristic. [Latin]
E.g. age, birth date,

5. What is Cohort ?

Cohort is a group of people who share common
characteristic or experience within the defined
period of time.

E.g. birth cohort, marriage cohort, Exposure
cohort, Synthetic cohort etc.

6. Cohort studies







longitudinal
Prospective studies
Forward looking study
Incidence study
starts with people free of disease
assesses exposure at “baseline”
assesses disease status at “follow-up”

7. 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 ample funds are available

10. C ohort Desig n
Study
population
free of
disease
Factor
present
Factor
absent
present
time
Study begins here
disease
no disease
disease
no disease
future

11. Cohort Studies
Disease
Population
People
without
disease
Exposed
No disease
Not
exposed
Disease
No disease

12. Frame work of Cohort studies
Disease Status
Total
Exposure
Status
Yes
No
a+b
c+d
N
Yes
a
No
b
c
d
a+c
b+d
Study
cohort
Comparison
cohort

13. Types of Cohort Study



Prospective cohort study
Retrospective (historical) cohort study
Combination of Retrospective and
Prospective cohort study.

14. Prospective cohort study
Exposure
Study starts
Disease
occurrence
time
Study starts
Exposure
Disease
occurrence
time

15. Retrospective cohort studies
Exposure
Disease
occurrence
Study starts
time

16. Cohort Studies

17. General consideration while
selection of cohorts




Both the cohorts are free of the disease.
Both the groups should equally susceptible to
disease
Both the groups should be comparable
Diagnostic and eligibility criteria for the
disease should be defined well in advance.

18. Elements of cohort study
1.
2.
3.
4.
5.
Selection of study subjects
Obtaining data on exposure
Selection of comparison group
Follow up
Analysis

19. Selection of study subjects

General population



Whole population in an area
A representative sample
Special group of population

Select group


occupation group / professional group
Exposure groups

Person having exposure to some physical, chemical or biological
agent

e.g. X-ray exposure to radiologists

20. Obtaining data on exposure

Personal interviews / mailed questionnaire

Reviews of records


Dose of drug, radiation, type of surgery etc
Medical examination or special test

Blood pressure, serum cholesterol

21. Obtaining data on exposure

Environmental survey


Water, Air, Sanitation status etc.
By obtaining the data of exposure we can
classify cohorts as


Exposed and non exposed and
By degree exposure we can sub classify cohorts

22. Selection of comparison group

Internal comparison



Only one cohort involved in study
Sub classified and internal comparison done
External comparison


More than one cohort in the study for the purpose
of comparison
e.g. Cohort of radiologist compared with
ophthalmologists

23. Selection of comparison group

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

24. Follow-up

To obtain data about outcome to be
determined (morbidity or death)





Mailed questionnaire, telephone calls, personal
interviews
Periodic medical examination
Reviewing records
Surveillance of death records
Follow up is the most critical part of the study

25. Follow-up

Some loss to follow up is inevitable due to death
change of address, migration, change of
occupation.

Loss to follow-up is one of the draw-back of the
cohort study.

26. ANALYSIS

Calculation of incidence rates among
exposed and non exposed groups

Estimation of risk

27. Incidence rates of outcome
Disease Status
Yes
Exposure
Status
Yes
No
a
c
No
b
d
a+c b+d
Total
a+b
c+d
N
Study
cohort
Comparison
cohort

28. Incidence rate


Incidence among exposed =
a
a+b
Incidence among non-exposed =
c
c+d

29. Estimation of risk

Relative Risk
incidence of disease among exposed
RR = ______________________________
Incidence of disease among non-exposed
a/a+b
=
_________
c/c+d

30. Estimation of Risk

Attributable Risk
Incidence of disease among exposed –
incidence of disease among non exposed
AR = _______________________________
Incidence of disease among exposed

31. Attributable Risk
a/a+b – c/c+d
AR = _______________
a/a+b

32. 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

33. Incidence Rates

Incidence of lung cancer among smokers
70/7000 = 10 per 1000

Incidence of lung cancer among non-smokers
3/3000 = 1 per thousand

34. Relative Risk and Attributable Risk
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)

35. Strengths of Cohort studies





Can establish population-based incidence
Accurate relative risk (risk ratio) estimation
Can examine rare exposures (asbestos > lung
cancer)
can establish cause - effect
minimizes selection and information bias

36. Strengths of Cohort studies
•
Can be used where randomization is not
possible
•
Magnitude of a risk factor’s effect can be
quantified
•
More than one disease related to single exposure
•
Multiple outcomes can be studied (smoking >
lung cancer, COPD, larynx cancer)

37. Weaknesses of Cohort studies






Often requires large sample
long time to complete
expensive
Ethical issues
Non response, migration and loss-to-followup biases
Unexpected environmental changes may
influence the association

38. THANK YOU

39. Examples
Presentation of cohort data:

40. 1. Population at risk
Does HIV infection increase risk of developing TB
among a population of drug users?
Population
Cases
(follow up 2 years)
HIV +
HIV -
215
289
Source: Selwyn et al., New York, 1989
8
1
EPIET (www)

41. Does HIV infection increase risk of
developing TB among drug users?
Exposure
Population
(f/u 2 years)
Cases
Incidence
(%)
Relative
Risk
HIV +
215
8
3.7
11
HIV -
298
1
0.3
EPIET (www)

42. 2. Person-years at risk
Tobacco smoking and lung cancer, England & Wales, 1951
Person-years
Cases
102,600
133
Smoke
Do not smoke
42,800
3

43. Various exposure levels

44. Cohort study: Tobacco smoking and
lung cancer, England & Wales, 1951