4. COHORT STUDY
COHORT
A group of people who share a common characteristic or experience within a defined period of time. e.g. age, occupation, exposure to drug, exposure to a vaccine, exposed to an infection, insured persons, smokers, obese, pregnant women, babies born on a single day or in a year.

5. COHORT STUDY
A group of individuals that is exposed to a risk factor (study factor) is compared with a group of individuals not exposed to a risk factor (control group).

6. Cohort study Cohort study usually proceeds from cause to effect.
Exposure has occurred but the disease has not.
In Cohort Study:
Study group is identified prior to appearance of disease.
Control group is identified prior to the appearance of disease.
Study groups are observed over a period of time to determine the frequency of disease among them.

7. Cohort study Risk factor Disease yes no total Positive a b a+b
Negative c d
c+d

8. Cohort study Cohort must be free from disease under study
Both study and control groups must be equally susceptible to disease under study
Both the groups should be comparable in respect to all possible variables
Diagnostic and eligibility criteria of the disease must be defined beforehand.
Groups must be followed under same identical conditions over a period of time to determine outcome of exposure.

9. Cohort study Risk factor Disease yes no total Positive a b a+b
Negative c d
c+d

10. Cohort study After end of follow up We can calculate
Incidence rate among exposed

11. Cohort study After end of follow up We can calculate
Incidence rate among exposed
= a / a + c

12. Cohort study After end of follow up We can calculate
Incidence rate among exposed
= a / a + c
Incidence rate among non exposed
=c / c + d

13. Cohort study After end of follow up We can calculate
Incidence rate among exposed
= a / a + b
Incidence rate among non exposed
=c / c + d
Cohort study is most reliable mean of showing an association between a suspected risk factor and subsequent disease.

14. Cohort study Elements of cohort study Selection of study subject
Obtaining data on exposure
Selection of comparison group
Follow up
Analysis

15. STEPS IN CCS Selection of cases and controls Matching
Measurement of exposure and
Analysis and interpretation

16. 1.Selection of study subjects
General population
Special group
Select groups------homogeneous group
Exposure group---experience of exposure
Physical
Chemical
Disease agent

17. 2. Obtaining Data On Exposure
Cohort members
Personal interviews
Mailed questionnaire
Review of records
Medical examination or special tests
Environmental surveys
Information required for:
Age, sex, gender & demographic variables etc.

18. 3. Selection of comparison group
Internal comparisons
External comparisons
Comparison with general population rates

19. 4. Follow up Regular follow up of all participants
Methods should be devised to measure outcome
Procedures required are;
Periodic medical examination of each member
Reviewing of physician, medical & hospital record
Routine surveillance of death records
Mailed questionnaire, telephone calls, periodic home visits etc.

20. 5.Analysis Incidence rates of outcome Estimation of risk Among exposed
Among non-exposed
Estimation of risk
Relative risk
Attributable risk

21. CIGARETTE SMOKING AND LUNG CANCER
Developed Lung cancer
Did not develop Lung cancer
Total
Yes No 70
(a) 3
( c)
6930
(b)
2997
(d)
7000
(a+b)
3000
(c+d)

22. 1.Incidence rates
Among exposed = a/ a+b=70/7000
= 10 per 1000

23. 1.Incidence rates Among exposed = a/ a + b = 70 / 7000
= 10 per 1000
Among non exposed= c/ c + d =3 / 3000
= 1 per 1000
P<0.001

24. Cohort study (CHD) Risk factor Disease yes no total Bankers a 60 b 40
100
Non bankers c 30 d 70
c+d

25. Cohort study (HTN) Risk factor Disease yes no Total obese a 100 b 900
1000
Non obese c 33 d
967
c+d

26. 2.ESTIMATION OF RISK Relative risk incidence among exposed
RR= _________________________
incidence among non exposed
RR=10/1=10(smokers are 10 times greater risk of developing Ca Lung)
It is a direct measure of strength of association between suspected cause and effect.
RR=1 no association
RR=>1 positive association b/w expos & dis.

27. ATTRIBUTABLE RISK
Is the difference in incidence rates of disease between an exposed group and non exposed group. It is expressed as percent.
incidence of disease rate among AR=exposed—incidence of disease rate among non exposed
incidence rate among exposed
AR=10-1/10X100=90% (90% of lung cancer was due to smoking).

28. Advantages of Cohort study
Incidence can be calculated
Several possible outcomes related to exposure can be studied
Direct estimate of relative risk
Dose response can be calculated
Minimized bias

29. Disadvantages of cohort study
Large no. of people are involved
Takes long time to complete the study
Administrative problems, loss of staff
Loss of original cohort
Selection of comparison groups
Change in standard methods
Expensive
Change in people behaviour

30. Differences between case control and cohort studies
Case control study
Cohort study
1. Proceeds from “effect to cause
Proceeds from “cause to effect”
2. Starts with the disease
Starts with people exposed to risk factor or suspected cause.
3. Tests whether the suspected cause occurs more frequently in those with the disease than among those without the disease.
Tests whether disease occurs more frequently in exposed, than in those not similarly exposed.
4. Usually the first approach to the testing of a hypotheses, but also useful for exploratory studies
Reserved for testing or precisely formulated hypothesis
5. Involves fewer number of subjects
Involves larger number of subjects
6. Yields relatively quick results
Long follow-up period
7. Suitable for the study of rare diseases
Inappropriate when the disease or exposure under investigation is rare.
8. Generally yields only estimate of RR (odds ratio)
Yields incidence rates. RR as well as AR.
9. Cannot yield information about diseases other than that selected for study
Can yield information about more than one disease outcome.
10. Relatively inexpensive
Expensive