Cohort ppt

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
o
h
o
r
t
D
e
s
i
g
n
C
o
h
o
r
t
D
e
s
i
g
n
time
Study begins here
Study
population
free of
disease
Factor
present
Factor
absent
disease
no disease
disease
no disease
present
future

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

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

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

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

15. Retrospective cohort studies
Exposure
time
Disease
occurrence
Study starts

16. Cohort StudiesCohort 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.Selection of study subjects
2.Obtaining data on exposure
3.Selection of comparison group
4.Follow up
5.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
N
dc
ba
Yes No
Disease Status
Yes
No
Exposure
Status
a+b
c+d
b+d a+c
Total
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 of lung cancer among smokers
70/7000 = 10 per 1000
Incidence of lung cancer among non-smokers
3/3000 = 1 per thousand
Incidence Rates

34. 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)
Relative Risk and Attributable Risk

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-follow-
up biases
Unexpected environmental changes may
influence the association

38. THANK YOU

39. Examples
Presentation of cohort data:

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

41. Exposure
Population
(f/u 2 years)
Cases
Incidence
(%)
Relative
Risk

HIV +

215

8

3.7

11
HIV - 298 1 0.3

Does HIV infection increase risk of
developing TB among drug users?
EPIET (www)

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

43. Various exposure levels

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