Cohort is defined as a group of people who share a common characteristic or experience within a defined time period.

1. COHORT STUDY
BY DR. Pankaj Sharma

2. CONCEPT OF COHORT
• Cohort is defined as a group of people who share a common
characteristic or experience within a defined time period.
TYPES OF COHORT:
• CLOSED/FIXED: Fixed group of persons followed from a certain point of time
to defined end population
• OPEN/DYNAMIC: Persons may leave/enter the study at any point without
compromising study design.

3. CHOICE OF STUDY COHORTS:
1. They have undergone some unusual exposure/experience of which the effects
are to be evaluated
2.Often come across some special resource that may facilitate ascertainment of
their exposure, follow up or disease experience.

4. DISTINGUISH FEATURES OF COHORT STUDY:
• Cohorts identified prior to appearance of disease under investigation
• Study group defined observed over a period of time to determine
frequency of disease among them
• Cause Effect
• Cohort study which is usually undertaken to obtain additional
evidence to refute/support existence of an association between
suspected cause and disease.

5. DEFINITION:
• It is also called as prospective cohort study or concurrent
cohort study or longitudinal study or forward looking study.
• It is concurrent because investigator identifies the original
population at the beginning of study and in effect,
accompanies the subjects concurrently through calendar
time until at which disease develops or does not develop.

6. • Prospective study
• Retrospective cohort study
• Ambi-directional cohort
study
TYPES OF COHORT STUDY

7. • The common strategy of cohort studies is to start with a
reference population, some of whom have certain
characteristics or attributes relevant to the study(exposed)
with others who don’t have thosecharacteristics(unexposed). Both
groups should be free from conditions under consideration.
• Investigator accompanies the subjects concurrently through
calendar time until the disease develops or not.
PROSPECTIVE COHORT STUDY

8. • A historical cohort study depends upon availability of data
or records that allow reconstruction of the exposure of
cohorts to a suspected risk factor and follow up of their
mortality or morbidity over time.
• Although the investigator was not present when exposure
was first identified, he reconstructs exposed and unexposed
populations from records and then proceeds as though had
been present throughout study.
• Outcome is ascertained at the time of study.
RETROSPECTIVE / HISTORICAL COHORT STUDY

9. DISADVANTAGESOFRETROSPECTIVECOHORTSTUDY:
All relevant
variables may
not available
in records
Difficult to
ascertain
study
population
was free of
disease
Attrition :loss
of records,
incomplete
records,
difficult in
tracing
original
population
Difficult to
obtain
reliable
information

10. INDICATION FOR COHORT STUDIES:
• There is good evidence of an association between exposure and
disease(derived from clinical observation and supported by
descriptive and case-control study)
• Exposure –rare, incidence –high
• Attrition of study can be minimized. (follow up easy, cohort stable
cooperative accessible)
• Ample fund

11. GENERALCONSIDERATION:
• Cohort must be free from disease
• Both groups should be equally susceptible to the disease
• Both groups should be comparable in all aspect
• Diagnostic and eligibility criteria must be defined before hand
• Groups followed under the same identical condition over a period of
time to determine outcome

12. DESIGN:
• Selection of cohort
• Collection of data on the
exposure
• Selection of comparsion
population
• Follow up
• Analysis

13. SELECTIONOFCOHORT:
• A community cohort of specific age and sex
• An exposure cohort e.g. smokers, users of OCP, radiologists
• A birth cohort, e.g. school entrants
• An occupational cohort, e.g. miners
• A marriage cohort
• A diagnosed cohort or treated cohort, e.g. cases treated with radiotherapy,
surgery.
The usual procedure is to locate or identify the cohort which may be
a total population in an area or sample thereof.
If study start with exposed and non exposed, it can explore only specific
exposure. If
study start with defined population it can explore multiple exposure.

14. DATACOLLECTION
• In a conventional cohort study, an initial cross sectional study is often
performed to exclude persons with the outcome of interest(disease)
and to identify the cohort that is free from the disease.
Data collected from
• cohort members: interviews/mailed questionnaires
• Medical records
• Medical examination
• Environmental survey

15. SELECTIONOFCOMPARSIONGROUP:
• Internal Control Group
• Exposed and non-exposed in the
same Study population
(Framingham study)
• Minimise the differences
between exposed and non-
exposed
• External Control Group
• When information on degree of
exposure is not available chose
another group, another cohort
(radiologists cohort with a cohort
of ophthalmologist)
General Population :
If none of the above comparison is
available than the mortality
experience of the exposed group is
compared with the mortality
experience of the general
population in the same geographic
area as the exposed people.
E.g. comparison of frequency of
cancer among uranium mine
workers with the rate in general
population in same geographic
area.
Limitation: non availability of
outcome data, difficulty in
selecting study and control

16. Ideal comparsion:
• A group of thymic enlargement at similar stage with similar clinical
profile who were not irradiated to rule out thymic enlargement
themselves were responsible for any usual outcome.
• Rarely available in non experimental study
• On the basis of indication and contra indication: introduce confounding
of the effects of treatment by its own
indication/contraindication(confounding by indication)