1. Epidemiology MPH 531 Analytic Epidemiology Cohort Studies
Mpundu MKC
MSc Epidemiology and Biostatistics, BSc Nursing, RM, RN

2. Cohort Studies Type of Analytic study
Unit of observation and analysis: Individual (not group)
Also called follow-up studies, incidence studies, longitudinal studies, or prospective studies

3. Cohort Studies At baseline (1st observation point):
Subjects are all disease free
Exposure is used to classify subjects into exposed or unexposed groups
Subjects are followed up to document incidence (occurrence) of disease

4. Cohort Studies
Two groups are selected, one of people with the Exposure, and the other of people with the same general characteristics but without Exposure (controls)
Follow up of study participants for the outcome
Compare the both groups to determine who developed the disease

5. Cohort Studies Direction Subjects selected on the basis of Exposure
Start with persons having Exposure
Look forward to development of Disease
Compare frequency of Disease in exposed with frequency of Disease in controls (non-exposed)

6. Cohort Studies
Are exposure and disease linked? E D B
Exposure
Disease

7. Cohort Studies Exposure ( 𝑬 + ) No Disease Source Population Disease
Controls ( 𝐸 − )

9. e.g. of Exposures in Cohort Studies
Exposures included:
Smoking
Alcohol use
Substance use
Unhealthy diets
Elevated blood pressure
physical inactivity

10. Measure of Association (Relative Risk (RR)
A ratio that measures the risk of disease among the exposed to the risk among the unexposed
RR Numerator: Incidence rate in the exposed
RR Denominator: Incidence rate in the unexposed

11. Types of Cohort Studies
Prospective
Exposure baseline in the present
Follow-up period: present to future
Retrospective
Exposure baseline in the past
Follow-up period: past to present

12. Types of Cohort Studies
3. Historical prospective or ambi-spective
Exposure baseline in the past
Follow-up period: past to present to future

13. Types of Cohort Studies
DESIGN
PAST
PRESENT
FUTURE
Prospective E D
Retrospective E D
Historical prospective E E D

15. Measure of association (Relative Risks)
Outcome
Exposure
𝑫 +
𝑫 −
Exposed a b
Not Exposed c d

16. Measure of association (Relative Risks)
Risk for group of primary interest
RR =
Risk for comparison group
Risk of illness among exposed = 𝑎 𝑎+𝑏
Risk of illness among non exposed = 𝒄 𝒄+𝒅

17. e.g. Calculating the Relative Risk
Disease Status
CHD cases
(Cases)
No CHD
(Controls)
TOTAL
Exposure Status
Smoker
112
176
288
Non-smoker 88
224
312
A/(A+B)
112 / 288
Relative Risk = = =
1.38
C/(C+D)
88 / 312

18. e.g. Interpreting the Relative Risk=
1.38
The risk of developing CHD is 1.38 times higher for a smoker than for a nonsmoker. or
The risk of developing CHD is 13.8% higher for a smoker than for a nonsmoker.

19. Outcome Measures in Cohort Studies
Incidence in the exposed
Incidence in the unexposed
Relative risk
Attributable risk (risk difference)
Population attributable risk
Attributable risk percent
Population attributable risk percent
Standardized mortality ratio

20. Advantages of Cohort Studies
Temporality: Exposure precedes outcome because the cohort is disease free at baseline
Efficient for studying rare exposures
May be used to study multiple outcomes
Allows for calculation of incidence of diseases in exposed and unexposed individuals
Minimizes recall bias

21. Disadvantages of Cohort Studies
Tend to be expensive (large sample size) and time consuming (long follow-up period)
Loss to follow-up
When multiple outcomes or specific disease incidence is the outcome of interest, bias can be a serious problem
Inefficient to study rare diseases

22. Cohort Studies Strengths of Cohort studies
Measures incidence rate and risk
More than one disease related to single exposure
Can establish cause - effect
Good when exposure is rare
Minimizes selection and information bias

23. Cohort Studies Weaknesses Loss to follow-up
Often requires large sample
Ineffective for rare diseases
Long time to complete
Expensive
Ethical issues