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COHORT STUDIES

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OPEN - people moving in and out. CLOSED - fixed population. ADVANTAGES OF COHORT STUDIES ... Benzene-workers (leukemia) Coke-oven workers (lung cancer) ... – PowerPoint PPT presentation

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Title: COHORT STUDIES


1
COHORT STUDIES
  • Nigel Paneth

2
TYPES OF COHORT STUDIES
  • A. TIMING
  • B. SAMPLING
  • C. POPULATION BASE
  • D. OPEN AND CLOSED COHORTS

3
TIMING
  • PROSPECTIVE (OR CONCURRENT)
  • RETROSPECTIVE (OR NON-CONCURRENT)

4
SAMPLING
  • cohort studies with sampling unrelated to
    exposure (common)
  • cohort studies with exposure-based sampling
    (rare)

5
POPULATION BASE
  • Population-based
  • not-population based

6
OPEN AND CLOSED COHORTS
  • OPEN - people moving in and out
  • CLOSED - fixed population

7
ADVANTAGES OF COHORT STUDIES
  • 1. Can assess several outcomes
  • 2. Time-order generally clear
  • 3. Prospective control over exposure and outcome
    measurement possible (in prospective studies)
  • 4. Somewhat less potential for bias than
    case-control studies, but equal potential for
    confounding

8
DISADVANTAGES OF COHORT STUDIES
  • 1. Generally require large samples
  • 2. Not useful for rare outcomes
  • 3. As an observational study, can never be
    assumed to be free of confounding and bias
  • 4. Must usually control for potential confounding
    in the analysis, though can control in the design

9
STUDY BASE IN COHORT STUDIES
  • The study base is the person-time experience of
    the individuals in whom the outcome is
    ascertained.
  • Study base should be the same for exposed and
    unexposed in cohort studies. Thus the
    denominator for calculation of incidence rates
    must be the same number of people for the same
    period of observation.
  • Calculation of person-years at risk is the
    means of achieving equivalence of study base in
    cohort studies.

10
POPULATION AT RISK IN COHORT STUDIES
  • 1. At baseline, we start with a cohort free of
    disease
  • 2. If analysis is by cumulative incidence, then
    denominator is study population at baseline.
  • 3. If analysis is by incidence density, then
    denominator is person-years at risk.

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15
FAMOUS COHORT STUDIES
  • POPULATION-BASED
  • CARDIOVASCULAR
  • CHILD HEALTH
  • SPECIAL EXPOSURES
  • NON-POPULATION BASED
  • OCCUPATIONAL for convenience
  • OCCUPATIONAL to study the occupation
  • HEALTH CARE SETTINGS
  • VETERANS

16
1. CARDIOVASCULAR DISEASE
  • Framingham, MA 
  • Tecumseh, MI 
  • Evans county, GA (biracial)
  • Muscatine, IA
  • Bogalusa, LA (children)

17
  • 2. CHILD HEALTH
  •  
  • National Birthday Trust Studies
  • One week of births in England and Wales in 1946,
    1958 and 1970
  • Project on Premature Infants
  • All births lt 1,500 g or lt 32 weeks in Holland in
    1983
  • The National Childrens Study http//www.nichd.nih.
    gov/about/despr/despr.htm
  • Will we shortly begin a study in the US of
    100,000 pregnancies with offspring followed to
    age 21?

18
  • 3. SPECIAL EXPOSURES
  •  
  • Atomic Bomb Casualty Commission (ABCC)
  • Hiroshima and Nagasaki survivors (effects of
    radiation)
  • Dutch famine survivors (effects of starvation)
  • Seveso (effects of dioxin exposure)

19
1. OCCUPATION-BASED COHORTSBECAUSE OF CONVENIENT
FOLLOW-UP
  • British Doctors Study
    (Doll smoking)
  • Nurses Study
    (Speizer, Willett many issues)
  • London civil servants
    (Marmot - SES)
  • Taiwanese civil servants
    (Beasley liver cancer)

20
2. OCCUPATION BASED TO STUDY EXPOSURES
  •   Benzene-workers (leukemia)
  • Coke-oven workers (lung cancer)
  • Asbestos workers (lung cancer)
  • Radium dial painters (oral cancer)

21
  • 3. SAMPLING FROM HEALTH CARE SETTINGS
  • National Collaborative Perinatal Project
  • Almost all pregnancies at 12 medical centers
    1959-1966 N about 50,000. (causes of CP)
  • Child Health and Development Studies
  • Kaiser-Permanente births (many issues)
  • Patients treated with radiation for polycythemia
    or ankylosing spondylitis (radiation and cancer)

22
  • 4. VETERANS
  • Mustard-gas poisoning from WW I (lung disease)
  • Vietnam Veterans (post-traumatic stress disorder,
    agent orange effects)
  • Gulf War Veterans
    (Gulf war syndrome)

23
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24
CASE-COHORT DESIGN PURPOSE
  • The case-cohort design is used to reduce the
    costs of exposure assessment

25
CASE-COHORT DESIGN APPROACH
  • 1. A population at risk is identified and
    screened for disease, and prevalent cases are
    omitted.
  • 2. A case-identification procedure is developed
    to detect new cases of disease in the cohort.
  • (so far all is the same as any cohort study)

26
CASE-COHORT DESIGN APPROACH
  • 3. The whole cohort is subject to
    case-identification, but only a random sample
    (called the sub-cohort) receives detailed
    exposure assessment.
  • 4. The cases are those emerging in the population
    (both in and out of the sub-cohort) the controls
    are subjects in the sub-cohort who are not
    cases.
  • 5. Analysis is like a cohort study. Since the
    sampling fraction is known, and the entire
    population is sampled for caseness, true
    incidences and relative risks can be calculated.
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