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Epidemiology
Cohort Studies (T Pierce)
16
Health Care
Graduate
10/22/2010

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Cards

Term
define cohort
Definition
group that shares common experience within a defined time period
Term
define cohort study
Definition
  • study where one or more groups of people that are free of disease and that differe according to extent of exposure (ex: exposed and unexposed) are compared with respect to disease incidence

Unlike case control, we start with exposure (case control starts with disease)

Term
Cohort studies- process and design
Definition
  • we ID exposed and nonexposed comparable groups and follow forward in time for the development of disease (NOT randomly assigned to exposed and unexposed group- it happens naturally)
  • may study morbidity and mortality
  • design can be retrospective, prospective, or ambidirectional
Term
cohort studies: prospective vs. retrospective
Definition

Retrospective studies generally rely heavily on records; this makes them dependent on the completeness and detail in existing records
Prospective studies are usually able to collect contemporaneous data specifically for the study
Other methods are basically the same
Retrospective cohort studies are more common
Term
Advantages and disadvantages of retrospective cohort
Definition

  • Advantages
    • Usually cheaper and faster to complete
    • Efficient with diseases with long latent period
  • disadvantage- Exposure data may be inadequate
Term
Advantages and disadvantages of prospective cohort
Definition

  • Disadvantages
    • More expensive and time consuming
    • Not efficient for diseases with long latent periods
  • Advantages
    • Better exposure and confounder data (easier to classify exposure)
    • Less vulnerable to  selection bias
Term
basic plan of cohort
Definition

1.   Identify cohort based on exposure or a common experience

2.   Follow entire population forward in time

Determine morbidity and/or mortality status of each individual in the cohort

3.   Choose unexposed population for comparison or members of cohort with differing degrees of exposure

4.   Compare rates of disease or mortality in exposed group to rates in unexposed/less exposed group

Term
cohort studies: how you ID cohort based on exposure or common experience
Definition

  • Must enumerate (identify) entire population
  • Methods to identify population include:
    • birth records
    • union records
    • Occupational or work histories
    • personnel records
    • census or other residence records
    • household survey (complete)
    • other records, e.g. school, military
  • Characterize exposure variation within cohort  (may be homogeneous or heterogeneous)
Term
plan of cohort studies: determine mortality status
Definition
  • determine vital status for each cohort memeber
  • determine cause of death for deceased indivuals
Term
cohort study: sources of exposure info
Definition
  • pre-existing records- inexpensive data recorded before disease occurence
    • disadvantages- detal can be inadequate, records can be missing, doesn't have confounders
  • questionarree interviews- good for info, but not routinely recorded but could have recall and interview bias
  • direct physical exams, tests, environmental monitoring may be needed to ascertain certain exposures
Term
cohort study: approaches to follow up
Definition
  • in any cohort study, ascertainment of outcome data involves tracing or following all subjects from exposure into future
  • many resources used
  • time consuming, but high losses to follow up raise doubt about validity of study
Term
cohort study- options for using unexposed population for comparison
Definition
  • general population rates aka external comparison group
  • use comparable group from outside you cohort (type of external comparison group, and is sometimes called a double cohort study) (It doubles size and cost)
  • divide cohort into different exposure groups and compare exposed to unexposed individuals aka internal comparison group
Term
measure used to analyze cohort study
Definition

relative risk = incidence rate among exposed/    incidence rate among unexposed

 

RR = ad/bc

Term
cohort studies: advantages and limitations
Definition
  • advantages
    • gives incidence rates
    • selection bias usually not a problem
    • can study rare exposures
    • can look at multiple outcomes
  • limitations
    • expensive, time consuming, long follow up, cant study disease with long latent period (PROSPECTIVE)
    • require large study population
    • if record based, there are often limited exposure data and usually no data on smoking, other confounders
    • not effective for studying rare diseases
Term
cohort studies: most concerning biases
Definition
  • healty worker effect
    • you expect to see lower SMR because in order to work, you need to be relatively healthy
  • exposure misclassification
  • outcome misclassification
  • ascertainment bias
Term
cohort study- measuring morbidity and mortality to external comparison groups
Definition
  • standardized mortality rates (SMR) used with general population comparison groups
  • standardized incidence ratio (SIR) and standardized rate ratio (SRR) generally used with other external comparison groups
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