COHORT STUDIES by sammyc2007

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									COHORT STUDIES


   Nigel Paneth
 TYPES OF COHORT
     STUDIES

A. TIMING
B. SAMPLING
C. POPULATION BASE
D. OPEN AND CLOSED COHORTS
        TIMING

• PROSPECTIVE
  (OR CONCURRENT)

• RETROSPECTIVE
  (OR NON-CONCURRENT)
         SAMPLING

• cohort studies with sampling
  unrelated to exposure (common)

• cohort studies with exposure-based
  sampling (rare)
POPULATION BASE


• Population-based

• not-population based
   OPEN AND CLOSED
      COHORTS


• OPEN - people moving in and out


• CLOSED - fixed population
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
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
           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.
    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.
FAMOUS COHORT STUDIES
1. POPULATION-BASED
 1. CARDIOVASCULAR
 2. CHILD HEALTH
 3. SPECIAL EXPOSURES
2. NON-POPULATION BASED
 1.   OCCUPATIONAL – for convenience
 2.   OCCUPATIONAL – to study the occupation
 3.   HEALTH CARE SETTINGS
 4.   VETERANS
1. CARDIOVASCULAR DISEASE

  •   Framingham, MA
  •   Tecumseh, MI
  •   Evans county, GA (biracial)
  •   Muscatine, IA
  •   Bogalusa, LA (children)
              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 < 1,500 g or < 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?
     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)
  1. OCCUPATION-BASED COHORTS
BECAUSE 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)
     2. OCCUPATION BASED
     TO STUDY EXPOSURES


•   Benzene-workers (leukemia)
•   Coke-oven workers (lung cancer)
•   Asbestos workers (lung cancer)
•   Radium dial painters (oral cancer)
 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)
           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)
CASE-COHORT DESIGN:
      PURPOSE


The case-cohort design is used to
   reduce the costs of exposure
           assessment
    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)
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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