1. Cohort Studies

2. Epidemiologic Studies
Identify new diseases
Identify populations at risk for a disease
Identify possible causative agents of disease
Identify factors or behaviors that increase risk of a disease

3. Study Designs
Means to assess possible causes by gathering and analyzing evidence.
The key to any epidemiologic study is in the definition of what constitutes a case and what constitutes an exposure.

4. Types of Study Designs Analytic Studies (to test hypotheses)
Experimental studies
Randomized clinical trials
Observational studies
Cross-sectional studies
Cohort studies (prospective study)
Case-control studies

5. What is Cohort?
Cohort is an ancient Roman military unit of men
A group of soldiers marching forward in battle

6. Cohort studies starts with people free of disease
longitudinal
Prospective studies
Forward looking study
Incidence study
starts with people free of disease
assesses exposure at “baseline”
assesses disease status at “follow-up”

7. Cohort Studies Study Population
Exposure is
self selected
Non-exposed
Exposed
Follow through
time
Disease
No Disease
Disease
No Disease

8. Cohort Study Examples
Study to determine if smokers have a higher risk of lung cancer
Study to determine if children who receive influenza vaccination miss fewer days of school
Study to determine if the Russian Salad was the cause of a foodborne illness outbreak

9. 2 x 2 Table
Used to summarize counts of disease and exposure in order to do calculations of association
Outcome/Disease
Exposure
Yes No
Total a b
a + b c d
c + d
a + c
b + d
a + b + c + d

10. 2 x 2 Tables a b c d a + b = total number who are exposed
a = number who are exposed and have the outcome
b = number who are exposed and do not have the outcome
c = number who are not exposed and have the outcome
d = number who are not exposed and do not have the outcome
*****************************************************
a + b = total number who are exposed
c + d = total number who are not exposed
a + c = total number who have the outcome
b + d = total number who do not have the outcome
a + b + c + d = total study population
Outcome
Yes No
Yes
Exposure No a b c d

11. Incidence among exposed
Relative Risk (RR)
It is the “ratio of incidence of disease among exposed to incidence of disease among non- exposed”
Incidence among exposed
Relative Risk = Incidence among not exposed

12. Relative Risk
The relative risk is the risk of disease in the exposed group divided by the risk of disease in the non-exposed group
RR is the measure used with cohort studies
Outcome
Yes No Total a
a + b
RR = c
c + d
Risk among
the exposed
Yes
Exposure No a b c d
a + b
c + d
Risk among
the unexposed

13. Relative Risk Example Food Poisoning a / (a + b) 23 / 33 RR = = = 6.67
Russian Salad
Yes No
Total 23 10 33 7 60 67 30 70
100
a / (a + b) / 33
RR = = = 6.67
c / (c + d) / 67

14. 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

15. 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

16. Framingham Study Design
Framingham, Massachusetts population was 28,000
Study started in 1948
Study design called for a random sample of 6,500
Enrollment questionnaire from targeted age range years
No clinical evidence of atherosclerotic cardiovascular disease
Cohort re-examined every two years

17. The Framingham Study Exposures included: Smoking Alcohol use Obesity
Elevated blood pressure
Elevated cholesterol levels
Low levels of physical activity, etc.

18. The Framingham Study Hypotheses:
Persons with hypertension develop CHD at a greater rate than those who are normotensive
Elevated blood cholesterol levels are associated with an increased risk of CHD
Tobacco smoking and habitual use of alcohol are associated with an increased incidence of CHD
Increased physical activity is associated with a decrease in development of CHD
An increase in body weight predisposes a person to CHD

19. Types of Cohort Studies
Concurrent Cohort Study (prospective or longitudinal)
Retrospective Cohort Study (historical cohort or non-concurrent prospective study)
Both designs are identical…comparing exposed and non-exposed populations
The only difference is calendar time.

20. Concurrent Cohort Study
Investigator identifies original study population at the beginning of the study.
The individuals are followed prospectively through time until disease develops or does not develop.
Disadvantages:
Requires long follow-up time (years)
Expensive
Age of investigator

21. Retrospective Cohort Study
The cohort is defined from historical data and followed up for disease up to the present time
Can telescope the frame of calendar time for the study and obtain results sooner
Disadvantage:
Quality depends on the historical data that are available – both to define exposure and to identify the outcome.

22. Assessment of Exposure
Techniques used to measure exposure include questionnaires (age, smoking habits), laboratory tests (cholesterol, hemoglobin), physical measurements (height, weight, blood pressure) and various special procedures (EKG, x-rays)
Quantifying exposures includes information such as date of onset, frequency of exposure and duration and intensity of exposure

23. Exercise
How would you design a cohort study of the association between preterm delivery and cigarette smoking?

24. Results
An exposed and a non-exposed group would first be identified e.g.) women presenting to a local county health department for prenatal care would be classified by smoking status – smokers and non-smokers.
These women would be followed to determine whether or not preterm delivery occurred.
The rates of the preterm delivery would be compared among the smokers and non-smokers.

25. Exercise : The RR
A cohort study was performed to study the association between hypercholesterolemia and CAD. There were 5000 people in the study, out of which 50% had the risk factor. Of these, 120 developed the disease after one year of follow-up. Only 2% of the non-exposed population developed the disease during the same time period. Calculate the RR.
STEPS:
Construct a 2 x 2 table and fill in data
Use the formula to calculate the RR
Check your result with mine
Interpret the result.

26. Exercise 2: 2 x 2 table Yes No Yes 120 2380 2500 2450 No 50 2500 170
CAD
Yes No
Yes
120
2380
2500
Hyper
Cholesterolemia
2450 No 50
2500
170
4830
5000

27. Exercise 2: Formula & Answer
The formula for calculating the RR is:
Incidence in exposed(Ie) = a / (a + b)
Incidence in unexposed (Iu) = c / (c + d)
Thus formula becomes
Ie / Iu = a / (a + b)
c / (c + d)
The Answer is
120 / 2500
50 / or
120 / 50 =
2.4
The answer of 2.4 means there is a 2.4 times greater risk of getting CAD in the hypercholesterol-emic group as compared to the group without hypercholesterolemia

29. To Review
The objective of the cohort study is to test a hypothesis regarding the causation of disease.
The group of persons to be studied (cohort) are defined in terms of characteristics manifest prior to appearance of the disease being investigated.
The defined study groups are observed over a period of time to determine and compare the frequency of disease among them.