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A cohort study is a type of observational investigation in which subjects are classified on the basis of the level of exposure to a risk factor and followed to determine subsequent disease outcomes.Cohort study characteristics
1. Observational study: No manipulation of the study factors
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2. Have a control group.
3. From ‘cause’ to ‘outcome’.
l “Prospective” = forward-looking in time
l Can be a historical perspective
l Also called follow-up, incidence, panel, or longitudinal study
4. Could examine the causal association between the exposure and the outcome.
Cohort study types
1. Prospective cohort study: The basic type of cohort study
2. Historical cohort study Or retrospective cohort study
3. Ambispective cohort study
Prospective cohort studies
Prospective cohort studies are conducted by making all observations on exposure and disease status after the onset of the investigation
Retrospective cohort studies
Retrospective cohort studies involve observations on exposure and disease status prior to the onset of the study
Cohort Study design and practice
· Selection of Subjects
The Exposed Group
The Unexposed Group
The Outcome Event
l Follow up
l Data Collection
Selection of exposed group
Exposure determining is based on a descriptive study and case-control study. The degree of exposure may differ depending on the goals of the study.
The investigator should identify an accessible population that is motivated to participate in the study and unlikely to discontinue participation
When the purpose of a cohort study is to investigate a community, such as in the Framingham Heart Study that community is the source of the unexposed persons.
Data Collection
Possible sources of data an exposure status include records, Interviews or Questionnaires, or Direct Measurements Made on Cohort Members.
Records
The available records include Occupational records, medical and pharmacy records, census records and so on
Direct Measurements
Many exposures of interest cannot be determined with any accuracy, or perhaps at all, for individual study subjects from either records or interviews, but can be determined by direct measurement.
Cumulative incidence rate
A number of new cases of disease occurring over a specified period of time in a population at risk at the beginning of the interval.
Cumulative Incidence
the probability (risk) of an individual developing the disease (outcome) during a specific period of time.
Incidence density
A number of new cases of disease occurring over a specified period of time in a population at risk throughout the interval. The probability (risk) of an individual developing the disease (outcome) during a specific period of time, using total person-time as the denominator. One subject followed one year contributes one person-year (PY).
Incidence density requires us to add up the period of time each individual was present in the population and was at risk of becoming a new case of the disease.
Incidence density characteristically uses as the denominator person-years at risk. (Time period can be person-months, days, or even hours, depending on the disease process being studied.)