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Cohort Studies. 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) .

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  • 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:
    • Cohortstudies
    • Case-control studies
    • Cross-sectional studies


  • 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
analytical studies
Analytical Studies
  • Control and experimental groups
  • Randomized groups
    • data collected without bias
  • Dependent and independent factors

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

Cohort Effect

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

Types of Cohort Studies

  • Prospective(concurrent)
  • Retrospective(historical)
  • Restricted(restricted exposures)

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

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

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.

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.

framingham study
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

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 30-59 years
  • No clinical evidence of atherosclerotic cardiovascular disease
  • Cohort re-examined every two years
  • Problems: white, middle class
hypothetical cohort study approach
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.

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
data gathering
Data Gathering
  • Person - to - person
  • Drop off questionnaire
  • Mailed to people
  • Telephone interview
  • Newsletter or magazine
potential biases in cohort studies
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.

advantages of prospective cohort studies
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
advantages of prospective studies cont d
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
disadvantages of prospective cohort studies
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
disadvantages of prospective studies cont d
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
survivorship studies
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
cohort life tables
(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.

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.

Is the Association Causal?

To be continued…