19 Flashcards
Analytic epidemiology:
Analytic epidemiology: exposures and outcomes
Observational:
observe people’s exposures and what happens to them
What is a cohort study?
“ …individuals are defined on the basis of presence or absence
of exposure to a suspected risk factor”
Cohort studies step by step
- Identify a source population
- Recruit your sample population (**Sample population must not already have the outcome of interest)
- Assess exposure to identify which group participants belong in (exposed or not exposed i.e. comparison)
- Follow up over time
- Observe whether or not participants develop the outcome
- Calculate measures of occurrence and measures of association
What can we measure using cohort studies?
Incidence proportion (IP)
Incidence rate (IR)
(Measures of occurrence)
Relative risk
Risk difference (attributable risk)
(Measures of association)
Some examples of cohort studies
What does PECOT stand for?
Population
Exposure
Comparison
Outcome
Time
- Identify a source population
- Recruit your sample population
What might we need to concider?
Ideal: random selection independent of exposure status
Sometimes: selection based on exposure status, (must consider appropriate comparison group)
Healthy worker effect
Can you be sure that the sample population does not already have the outcome?
- Assess exposure to identify which group participants belong
in (exposed or not exposed i.e. comparison)
What might we need to consider carefully?
Have participants been correctly classified?
- Follow up over time
What might we need to consider carefully?
Ø Have participants changed exposure status over time?
Ø Has everyone been followed up over the entire study?
Ø How long do participants need to be followed up?
- Observe whether or not participants develop the outcome
What might we need to consider carefully?
Has the outcome status been correctly classified?
Strengths of cohort studies
Determine temporal sequence between exposure and outcome
Can examine multiple outcomes from an exposure
Can calculate incidence (and therefore relative risk and risk difference)
Good for studying rare exposures
Limitations of cohort studies
Loss to follow up (Can lead to bias if related to the exposure and the outcome)
• Potential for misclassification of exposures/outcomes
• Generally not good for studying rare outcomes
• Time consuming
• Can be expensive
Two types of cohort studies
1 . Prospective cohort studies
2. Historical cohort studies (Sometimes called retrospective cohort studies)
The difference in starting point in prospective vs historical cohort studios
What is the difference between a prospective and retrospective cohort study?
The starting point
Features of a historical cohort study
• Use existing data
• Reconstruct follow-up period in the past
Pros and cons of historical cohort studies
PROS
• Less time consuming compared with prospective cohort studies
• Good for outcomes that take a long time to develop
• Less expensive
CONS
• Use existing data (collected for other reasons) – quality?
• May not know about all relevant factors
• Bias?
the difference between historical cohort studies, case-control studies and cross-sectional studies
historical cohort studies focus on a group with a shared past exposure and follow them into the future, case-control studies start with individuals who already have an outcome and compare them to controls to identify risk factors, and cross-sectional studies provide a snapshot of a population’s characteristics at a single point in time.
