Cohort Study

Question 1

What is a Cohort Study?

Answer 1

• A non-experimental study in which the unit of study is an individual
• Groups of exposed and non-exposed individuals followed for incidence of disease
  
  • You can start with populations of exposed and non-exposed (e.g. occupational cohort) and even select a specific number of subjects in defined exposure categories
  • You can start with defined population before members become exposed or exposures have been identified (e.g., community, professional/social organization)
• Need to have ideas of exposures expected to cause disease(s) of interest
  
  • Evidence obtained by cross-sectional, case-control or other types of studies
• Easier to conduct when the interval between exposure and disease is relatively short
• Quality of information needs to be the same in exposed and non-exposed
• Prospective: exposure and non-exposure ascertained during the study, groups followed for several years, incidence is measured.
• Retrospective: exposure from past records and outcomes, ie disease versus no disease is ascertained at time study has begun

Question 2

What are the measures of association in cohort studies?

Answer 2

• Allow Calculation of Incidence: Cohort studies allow you to calculate the incidence of disease in exposure groups, so you can calculate:
  
  • Absolute risk (incidence)
  • Relative risk (risk ratio or rate ratio)
  • Risk difference
  • Attributable proportion (attributable risk %)

Question 3

What are the pros of a cohort study?

Answer 3

• Can estimate incidence rate (and thus risk) in exposed and unexposed
• Good when exposure is rare
• Can study multiple outcomes
• Less bias in assessing the exposure (prospective study)
• No temporal ambiguity: exposure occurs before the disease outcome
• Clarity of Temporal Sequence (Did the exposure precede the outcome?):
• in a cohort study, subjects are known to be disease-free at the beginning of the observation period when their exposure status is established.
• In case-control studies, one begins with diseased and non-diseased people and then ascertains their prior exposures.
• Rare Exposures:
  
  • Exposure to toxic chemicals (Agent Orange)
  • Adverse effects of drugs (e.g., thalidomide) or treatments (e.g., radiation treatments for ankylosing spondylitis)
  • Unusual occupational exposures (e.g., asbestos, or solvents in tire manufacturing, )

Question 4

What are the cons in a cohort study?

Answer 4

• Large study population and long follow-up usually needed
• Generally expensive and logistically difficult
• Inefficient or not feasible for rare diseases
• Potential bias in assessing outcome if exposure status is known
• Changes in exposure status and in diagnostic criteria over time can affect classifications
• Non-participation and non-response (lost to follow-up) can significantly impact findings
  
  • To avoid bias, those that leave the study need to have the same probability of an event as those remaining in study
  • If sicker withdrew more, the study may eventually include a greater proportion of persons at lower risk
  • Incidence could be underestimated; survival could be overestimated

Question 5

What are limitations in retrospective cohort studies?

Answer 5

• If one uses records that were not designed for the study, the available data may be of poor quality.
• There is frequently an absence of data on potential confounding factors if the data was recorded in the past.
• It may be difficult to identify an appropriate exposed cohort and an appropriate comparison group.
• Differential losses to follow up can also bias retrospective cohort studies.

Question 6

What are the limitations in prospective cohort studies?

Answer 6

• You may have to follow large numbers of subjects for a long time.
• They can be very expensive and time consuming.
• They are not good for rare diseases.
• They are not good for diseases with a long latency.
• Differential loss to follow up can introduce bias.

Question 7

What are ambidirectional cohort studies

Answer 7

• A cohort study may also be ambidirectional, meaning that there are both retrospective and prospective phases of the study.
• Ambidirectional studies are much less common than purely prospective or retrospective studies, but they are conceptually consistent with and share elements of the advantages and disadvantages of both types of studies.