Cohort study design

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

2. 1. Define a prospective cohort
study

3. Cohort study design (Prospective)
Exposure Observer Disease
?
Start with two groups of people who are exposed and
unexposed, follow them to see who will get the
disease.

4. 1. Define a prospective cohort study
• Data on exposure status are available at
the beginning of the study, but we have to
wait for the outcome (disease).
• The disease will occur in the future!!
PROSPECTIVE

5. Exposure Study starts
Disease
occurrence
Prospective cohort study
EPIET (www)
time

6. 2.Define a retrospective cohort
study

7. Cohort study design (Retrospective)
Exposure Disease Observer
?
Start with two groups of people who are exposed
and unexposed, find out who got the disease.

8. Define a retrospective cohort study
• Historical cohort study
• both exposure and outcome (disease) have
already happened, but groups are still made
based on exposed or unexposed
• Investigator collected these data from the
medical documentations and records
• both exposure and disease took place in the
past RETROSPECTIVE

9. Retrospective cohort studies
Exposure
time
Disease
occurrence
Study starts
EPIET (www)

10. Cohort study
• The aim of the cohort study is to investigate whether
there is an association between exposure to a risk
factor and development of a disease.
• If an association exists, how strong is it?
• The value of RELATIVE RISK gives the answer!!!

11. Relative risk (RR)
Relative risk = Risk in exposed / Risk in non exposed
 Probability of developing a disease in exposed people
compared to the probability of developing a disease
in non exposed people.

12. Relative risk (RR)
RR = 1 Risk in exposed is the same like the risk in non exposed.
RR > 1 Risk in exposed is greater than risk in non exposed.
(positive association, possibly causal)
RR < 1 Risk in exposed is less than risk in non exposed.
(negative association, possibly protective)

13. Attributable risk (AR)
• Attributable risk (AR) is the difference between the
risk in exposed and risk in non-exposed
AR = Risk in exposed - Risk in non exposed
• AR is the portion of the risk (incidence/mortality of a
disease) in the exposed that is due to the exposure.

15. Etiologic fraction
EF= Risk in exposed - Risk in non exposed x 100
Risk in exposed
What portion of event (disease/death) can be avoided if
the exposure doesn’t exist?

16. 7. To what extent the avoidance of smoking
reduce the risk of death?
Mt (heavy smokers) = 18.84/1000
Mt (non smokers) = 13.25/1000
AR= 18.84/1000 – 13.25/1000 = 5.59/1000
Etiologic fraction =18.84/1000 – 13.35/1000 x100= 29.7%
18.84/1000
If the smoking doesn’t exist 29.7% of all deaths can be
avoided!

17. 8. Does the avoidance of smoke inhalation
reduce the risk of death and to what extant?
Mt (smokers who inhale fume) = 1.9/1000
Mt (smokers who not inhale fume) = 1.1/1000
EF = 1.9/1000 – 1.1/1000 x 100 = 42.1%
1.9/1000
Avoidance of smoke inhalation reduce the risk of deaths from
lung cancer for 42.1%.

18. Hiroshima
• RR = I (000-999)
I (2000-9999)
• RR = 1366/100,000
28/100,000
• RR = 48.8
People who were closest to the
epicenter had 48.8 times
higher chance to get
leukemia compared to those
who were farthest from the
epicenter.

19. Hiroshima
• RR = I (000-999)
I (2000-9999)
• RR = 1366/100,000
28/100,000
• RR = 48.8
Nagasaki
• RR = I (000-999)
I (2000-9999)
• RR = 563/100,000
37/100,000
• RR = 15.2

20. Comparison between case-control and cohort studies
Characteristics Case-control study Cohort study
Number of
participants
Duration of study
Cost of study
Choice of control
group
Reliability of data of
exposure

21. Comparison between case-control and cohort studies
Characteristics Case-control study Cohort study
Number of
participants
small large
Duration of study
Cost of study
Choice of control
group
Reliability of data of
exposure

22. Comparison between case-control and cohort studies
Characteristics Case-control study Cohort study
Number of
participants
small large
Duration of study short long
Cost of study
Choice of control
group
Reliability of data of
exposure

23. Comparison between case-control and cohort studies
Characteristics Case-control study Cohort study
Number of
participants
small large
Duration of study short long
Cost of study inexpensive expensive
Choice of control
group
Reliability of data of
exposure

24. Comparison between case-control and cohort studies
Characteristics Case-control study Cohort study
Number of
participants
small large
Duration of study short long
Cost of study inexpensive expensive
Choice of control
group
problem not a problem
Reliability of data of
exposure

25. Comparison between case-control and cohort studies
Characteristics Case-control study Cohort study
Number of
participants
small large
Duration of study short long
Cost of study inexpensive expensive
Choice of control
group
problem not a problem
Reliability of data of
exposure
problem not a problem

26. Comparison between case-control and cohort studies
Characteristics Case-control study Cohort study
Problems of attrition
Calculation of rates
Estimation of
relative risk
Effects of
measurement

27. Comparison between case-control and cohort studies
Characteristics Case-control study Cohort study
Problems of attrition not a problem problem
Calculation of rates
Estimation of
relative risk
Effects of
measurement

28. Comparison between case-control and cohort studies
Characteristics Case-control study Cohort study
Problems of attrition not a problem problem
Calculation of rates The rates cannot be estimated
unless the study is population
based.
The rates can be estimated.
Estimation of
relative risk
Effects of
measurement

29. Comparison between case-control and cohort studies
Characteristics Case-control study Cohort study
Problems of attrition not a problem problem
Calculation of rates The rates cannot be estimated
unless the study is population
based.
The rates can be estimated.
Estimation of
relative risk
The relative risk can not be
estimated. The risk can be
estimated only indirectly (OR-
odds ratio)
The relative risk can be
estimated.
Effects of
measurement

30. Comparison between case-control and cohort studies
Characteristics Case-control study Cohort study
Problems of attrition not a problem problem
Calculation of rates The rates cannot be estimated
unless the study is population
based.
The rates can be estimated.
Estimation of
relative risk
The relative risk can not be
estimated. The risk can be
estimated only indirectly (OR-
odds ratio)
The relative risk can be
estimated.
Effects of
measurement
Examine multiple
etiologic factors for a
single disease
Examine multiple effects
of a single exposure