1. Cohort study:
PROSPECTIVE STUDY,
LOGITUDINAL STUDY,
INCIDENCE STUDY &
LOOKING FORWORD STUDY.

2. FEATURES OF COHORT STUDY
The cohort are identified prior to the
appearance of the disease under
study
The study group so defined are
observed over a period of time to
determine the frequency of the
disease among them.
Cause to effect

3. Concept :
by definition, a group of people who share
a common characteristic or experience
within a defined time period.
E.g. age, occupation, exposure to a drug or
vaccine, pregnancy.
The comparison group may be general
population from which the cohort is drown,
or it may be another cohort of a persons
thought to have had little or no exposure to
the substance in question, but otherwice
similar

4. Indications:
When there is good evidence of an association
between exposure and disease, as derived from
clinical observations and supported by descriptive
& case control studies.
When exposure is rare, but the incidence of the
disease is high among the exposure group like
those in industries, exposure to x rays ect.
When attrition of the population can be minimized,
e.g. follow up is easy, cohort is stable, co
operative and easily accessible.
When ample funds are available.

5. Frame work
Effect to cause…… exposure has occurred
but the disease has not.
cohort Disease
Yes No
Total
Exposed to
Etiological
factor
a b a+ b
Not exposed to
Etiological
factor
c d c+ d

6. Types of cohort study:
Prospective cohort study
Retrospective cohort study
A combination of retrospective and
prospective cohort study

7. Three types of cohort study have
been distinguished on the basis of the
time of the occurrence of the disease
in relation to the time at which the
investigation is initiated and
continued.

8. Prospective cohort study/current
The out come not occurred at the
time the investigation begins
Most study are begin in present and
continue into future.
E.g. smoking and the lung cancer.

9. Retrospective study/ historical
The out come all occurred before the
start of the investigation.
the investigation goes back in time,
some times 10 yrs to 30 yrs.
to select these groups from existing
records of past employment, medical
records, ect.

10. A combination of retrospective and
prospective cohort study
The cohort is identified from the past
records, and it is assessed of date for
out come. The same record is
followed up prospectively in to the
future for further assessment.

11. ELEMENTS OF COHORT STUDY
SELECTION OF STUDY SUBJECTS
OBTAINNING DATA ON EXPOSURE
SELECTION OF COMPARISION
GROUPS
FOLLOW UP
ANALYSIS

12. SELECTION OF STUDY
SUBJECTS
GENERAL POPULATION
Special groups:
A) select groups:
B) Exposure group

13. OBTAINNING DATA ON
EXPOSURE
Cohort members: personal interview,
or mailed questionnaire
Review of the records
Medical examination or special test
Environmental survey

14. SELECTION OF COMPARISION
GROUPS
Internal comparison
External
Comparison with general population
rates

15. FOLLOW UP
Regular follow up of participant?
Depending on the out come to
determined. (morbidity or mortality).
Periodic medical examination of each
member of the cohort
Reviewing physical & hospital records
Routine surveillance of death records
Mailed questionnaire, telephone calls,
periodic home visits……

16. Drawback:
Migration, change of residence or
withdrawal from occupation.
These loses may bias the result

17. ANALYSIS
The data are analyze in terms of:
a) Incidence rates of outcome among
and non exposed.
b) Estimation of risk:
i) relative risk
ii) population attributed risk

18. Advantages
Incidence can be calculated
Several possible outcomes related to exposure can be
studied simultaneously---- i.e. we can study the association
of suspected factor
with many other diseases in addition to one under study
Cohort study provide a direct estimation of relative risk
dose response ratio can be calculated
Since comparison groups are formed before the disease
develops, certain forms of bias can be minimize like
misclassification of the individuals in to exposed and
unexposed groups.

19. Disadvantages
Large population, they are generally unsuitable for
investigating uncommon disease or disease with low
incidence in the population.
it takes long time to complete the study and to obtain the
result ( 20-30 yrs or more in cancer studies) by which time
the investigators may died or participants may have change
there classification. it is difficult to keep a large number of
individuals under medical surveillance indefinitely.
Administrative problems such as loss of funding, loss of
experienced staff, extensive record keeping are inevitable
Lose of interest, migration
Selection of comparison groups which are representative of
the exposed & unexposed segments of the population is a
limiting factor.
Expensive
Alter people’s behavior

20. case control cohort study.
Effect to cause
Start with the disease
Test the suspected cause
occurs more frequently in
those with the disease than
among those without the
disease
Usually the first approach to
the testing of a hypothesis,
but also useful for exploratory
studies
Involves fewer number of
subjects
Yields relatively quick results
Suitable for the study of rare
diseases
Relatively inexpensive
Cause to effect
Starts with the people
exposed to risk factor or
suspected cause
Test whether the disease
occurs more frequently in
those exposed, than in those
not similarly exposed
Reserved for testing of
precisely formulated
hypothesis
Involves large number of
subjects
Long follow up period often
needed, involving delayed
results.
Inappropriate when the
disease or exposure under
investigation is rare
expensive