1. Cohort Studies

2. Introduction
All studies involve some descriptive or analytic type of comparison of exposure and disease status.
Analytical study design options include: observational or interventional (which one is based on the role of the investigator).
There are three basic types of observational analytical study designs:
Cohort studies
Case-control studies
Cross-sectional studies

3. Introduction First, using observational analytical studies : Second:
Specific epidemiological study designs can be used to reveal etiologic (causal) relationships
First, using observational analytical studies :
Determine whether there is an association between a factor or a characteristic and the development of disease
Second:
From these associations, derive appropriate inferences regarding a possible causal relationship

4. Analytical Studies Control and experimental groups Randomized groups
data collected without bias
Dependent and independent factors

5. Cohort Studies Group by common characteristics
Start with a group of subjects who lack a positive history of the outcome of interest yet are at risk for it (cohort).
Think of going from cause to effect.
The exposure of interest is determined for each member of the cohort and the group is followed to document incidence in the exposed and non-exposed members.

6. When is a cohort study warranted?
When good evidence suggests an association of a disease with a certain exposure or exposures.

7. Cohort Effect
Changes and variation in the disease or health status of a study population as the study group moves through time.
“Generation effect”

8. Types of Cohort Studies
Prospective (concurrent)
Retrospective (historical)
Restricted (restricted exposures)

9. Types of Cohort Studies
Prospective – cohort characterized by determination of exposure levels (exposed vs. not exposed) at baseline (present) and followed for occurrence of disease in future
Groups move through time as they age
Retrospective - makes use of historical data to determine exposure level at some baseline in the past and then determine subsequent disease status in the present.
Restricted - limited exposure, narrow behavior (military, long shore men)

10. Prospective Studies Also called Looking into the future Example:
longitudinal
concurrent
incidence studies
Looking into the future
Example:
Framingham Study of coronary heart disease (CHD)

11. Design of a Cohort Experiment
The essential characteristic in the design of cohort studies is the comparison of outcome in an exposed group and a nonexposed group (or a group with a certain characteristic and a group w/o that characteristic).
A study population can be chosen by selecting groups for inclusion in the study on the basis of whether or not they were exposed

12. Selection of Cohort Groups
There are two basic ways to generate cohort groups.
Select a cohort (defined population) BEFORE any of its members become exposed or before the exposures are identified.
Select a cohort on the basis of some factor (e.g., where they live) and take histories (e.g., blood tests) on the entire population to separate into exposed and non-exposed groups.
Regardless of which selection approach is used, we are comparing exposed and non-exposed persons.

13. Design of a Cohort Experiment

14. Design of a Prospective Cohort Experiment
Major problem with a prospective cohort design is that the cohort must be followed up for a long period of time.

15. Framingham Study
Designed to study the effect of multiple factors on coronary heart disease (CHD):
age
hypertension
elevated blood cholesterol
tobacco smoking
increased physical activity
increase in body weight
diabetes mellitus

16. Framingham Study Design
Framingham, Massachusetts population was 28,000
Study design called for a random sample of 6,500
Enrollment questionnaire form targeted age range years
No clinical evidence of atherosclerotic cardiovascular disease
Cohort re-examined every two years
Problems: white, middle class

17. Hypothetical Cohort Study Approach
Examine people over their life times for contraction of diseases. In meantime, ask questions concerning diet, lifestyle,habits, work, etc.
Take blood and do lab tests (as many as possible).
Do same tests every year for a period of years.
Interventional aspect: If positive findings occur, they refer you to a specialist. By screening, they may have prolonged a life.

20. Sampling Valid, reliable surveys Critical number of subjects Randomize
Garbage in,
garbage out
Valid, reliable surveys
Critical number of subjects
the more, the better
Randomize
random selection
random assignment
Rule out bias
For example, degree of accuracy with which subjects have been classified with respect to their exposure.
For example, individuals who are sick may be more likely to give the kind of responses that they believe the investigator wants to hear

21. Data Gathering Person - to - person Drop off questionnaire
Mailed to people
Telephone interview
Newsletter or magazine

22. Potential Biases in Cohort Studies
Information bias
Bias in estimation of the outcome
Bias from non-response
Bias from losses to follow-up
Analytic bias
Your assignment: Describe and differentiate between these types of biases.

23. Advantages of Prospective Cohort Studies
Captive groups
Large sample sizes
Certain diseases or risk factors targeted
Can be used to prove cause-effect
Assess magnitude of risk
Baseline of rates
Number and proportion of cases that can be prevented

24. Advantages of Prospective Studies (cont’d)
Completeness and accuracy
Opportunity to avoid condition being studied
Quality of data is high
Considers seasonal and other variations over a long period
Tracks effects of aging process

25. Disadvantages of Prospective Cohort Studies
Large study populations required
not easy to find subjects
Expensive
Unpredictable variables
Results not extrapolated to general population
Study results are limited
Time consuming/results are delayed
Requires rigid design and conditions

26. Disadvantages of Prospective Studies (cont’d)
Subjects lost over time (dropouts)
Costs are high
Logistically demanding
Maintaining quality, validity, accuracy and reliability can be a problem

27. Survivorship Studies
Survivorship is the number of persons out of a study population who would survive until a certain time interval has been reached
Shows the chance that an event (such as death from cancer) will occur in successive intervals of time once a diagnosis has been made
Analysis yields a cumulative probability of surviving the projected time period
For infectious diseases, we use case fatality rate to assess survival
For chronic diseases, we use cohort life tables

28. (Cohort) Life Tables
Charts which summarize the patterns of survival and death in study groups of certain types of disease (chronic)
Insurance companies study these charts very closely.

29. Survival curves and risk of death for males vs
Survival curves and risk of death for males vs. females based on life tables in California for 1980.
Dip at beginning of life is due to infant mortality rate.
As one reaches the later years of life, the survival curve goes down and the risk of death goes up.

30. Is the Association Causal?
To be continued…