What is Cohort?
Indication and Elements of Cohort Study.
What is Relative risk and Attributable risk, and its interpretation?
Advantages & disadvantages of Cohort study.
Difference between Case control & Cohort study.
1. COHORT STUDY
Dr. Animesh Gupta
M.B.B.S., M.D., F.D.M., F.A.G.E.
Associate Professor
Department of Community Medicine, NMCH, Jamuhar
(Bihar)
2. SLO
At the end of this class, the students should know
✓ What is Cohort?
✓ Indication and Elements of Cohort Study.
✓ What is Relative risk and Attributable risk, and its
interpretation?
✓ Advantages & disadvantages of Cohort study.
✓ Difference between Case control & Cohort study.
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3. Epidemiology
Defined by John M. Last in 1988
“Study of distribution and determinants of health
related state or event in a specified population
and the application of this study to the control of
health problem”.
Measurement –
Disease frequency
Diseases distribution
Determinants of disease.
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4. Epidemiological Methods
OBSERVATIONAL STUDIES
DESCRIPTIVE STUDY ANALYTICAL STUDIES
✓ TIME
✓ PLACE
✓ PERSON
➢ ECOLOGICAL STUDY
➢ CROSS SECTIONAL STUDY
➢ CASE-CONTROL STUDY
➢ COHORT STUDY
EXPEREMENTAL STUDIES
➢ RANDOMIZED CONTROLLED TRIAL (RCT)
➢ FIELD TRIAL
➢ COMMUNITY TRIAL 4/25/2020Dr. Animesh Gupta Cohort Study
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5. WHAT IS COHORT?
Ancient Roman Military
unit, A band of warriors.
Persons banded
together.
Its a group of people
who share a common
characteristic or
experience. [Latin]
E.g. age, birth date,
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6. Cohort Study
Also k/a
Longitudinal
Prospective studies
Forward looking study
Incidence study
Starts with people free of disease
Assesses exposure at “baseline”
Assesses disease status at “follow-up”
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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
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8. 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.
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9. Design
To examine a relation between a factor and the
disease:
All subjects are free from the disease at the
beginning
Subjects are categorised on the basis of
presence or absence of exposure to the risk
factor
Subjects are then followed over time to
determine who develops the disease
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10. Study Design
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People
without
the
disease
Exposed
Not
Exposed
Disease
No
Disease
Disease
No Disease
11. Elements of cohort study
Selection of study subjects
Obtaining data on exposure
Selection of comparison
group
Follow up
Analysis
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12. Selection of study subjects
General population
Whole population in an area
A representative sample
Special group of population
Select group
occupation group / professional group (Dolls study )
Exposure groups
Person having exposure to some physical, chemical or
biological agent
e.g. X-ray exposure to radiologists
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13. 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
Environmental survey
✓ By obtaining the data of exposure we can classify
cohorts as
Exposed and non exposed and
By degree exposure we can sub classify cohorts
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14. 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
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15. 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
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16. 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
✓ Some loss to follow up is inevitable due to death change
of address, migration, change of occupation. 4/25/2020
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17. ANALYSIS
Calculation of incidence rates among exposed
and non exposed groups
Estimation of risk
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18. INCIDENCE RATES OF OUTCOME
a b a + b
c d c + d
a + c b + d
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EXPOSURE
STATUS
DISEASE STATUS
TOTAL
STUDY COHORT
COMPARISON COHORT
YES
NO
YES NO
❑ Incidence among exposed = a/ (a+b)
❑ Incidence among non exposed = c/ (c+d)
TOTAL
19. Relative risk
✓ It is the ratio of the incidence of disease in
exposed compared to incidence in non
exposed
RR= incidence among exposed
incidence among non exposed
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20. Interpretation
If RR=1.0, then there is no association between
the exposure and the disease
If RR>1.0, then there is positive association
between the agent and disease
If RR<1.0, then there is negative association,
which means a curative effect of the agent on
the risk of disease. Eg: immunisation lowers the
risk of disease.
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21. Attributable risk
“Risk difference”
It is the difference in the incidence rate among
exposed to that of non exposed.
AR=
incidence rate among exposed – incidence rate among non exposed
incidence rate among exposed
AR indicates to what extent the disease under study
can be attributed to the exposure
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22. Types of Cohort Study
Prospective cohort study
Retrospective (historical) cohort study
Combination of Retrospective and Prospective
cohort study.
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25. ADVANTAGES
Can establish cause-effect relationship
Can find out multiple diseases related to single
exposure
Can find the incidence rate and risk
Useful when exposure is rare
Minimises selection and information bias
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26. DISADVANTAGES
It often requires a large sample
Its ineffective for rare diseases
Losses due to follow up
Takes a long time for completion
Expensive
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27. Case Control Vs Cohort study
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Type Retrospective Prospective
Study design From outcome to exposure From exposure to
outcome
Indication When disease is rare When exposure is rare
Temporal association Not proven Proven
Duration Shorter or quickly longer
Sample size Less More
Outcome estimation Only odds ratio, neither
incidence or prevalence
Incidence as well as RR
Logistic efforts Cheap Expensive
Bias Recall, Berksonian Loss to follow up, cross
over bias