Lecture 19 Cohort study Flashcards
cohort study
allows Analytic epidemiology
Observational
Analytic epidemiology
- Examining observations between exposures and outcomes
- Causation
Observational
observe people’s exposures and what happens to them
what is a cohort study?
Individuals are defined based on the presence or absence of exposure to a suspected risk factor
Look at people who are exposed to potential risk factor follow over time to see if they develop the outcome interested in
cohort studies step by step use
GATE frame
- 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?
Measures of occurrence
- Incidence proportion (IP)
- Incidence rate (IR)
measures of association
- RR
- RD
Incidence proportion
Proportion of people who develop the disease during a time period
Specify time period
Incidence proportion calculation
of people who develop disease in a specific period / # of people at risk of developing disease at start of period
Incidence rate
Accounting for different lengths of time that people may be in the study
Measure of the rate of onset of the disease
Incidence rate calculation
of people who develop disease in a specific period / # of person years at risk of developing disease
Relative risk
Number of times is likely it is that the exposed grouped will develop the disease compared with the comparison group
Risk difference
Number of extra cases of that outcome there were in the exposed group compared to the comparison group
How many fewer cases there were
RR calculation
I exposed / I comparison
RD calculation
I exposed - I comparison
Recruit your sample population for cohort study
- Randomly selecting a single group of participants
- completely separate to whether the participants have the exposure or not (some will have exposure and some may not they will be classified accordingly)
- Sometimes study a rare exposure that generally appears in a particular group of people
Cohort studies recap
- Identify a source population
- Recruit your sample population
- Assess exposure to identify which group participants belong in
- Follow up over time
- Observe whether or not participants develop the outcome
- Calculate measures of occurrence and measures of association
What might we need to consider carefully?
- Identify a source population
- Recruit your sample population
- 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
Participants with rare exposures
Healthy worker effect can occur
Healthy worker effect
- If an occupational group is selected as the exposed group but the comparison group is selected from the general population
Selection bias
Problem
- People who are working are healthier but some people in the population includes people who are unable to work (selection bias)
Participants must not already have what at the start of the study?
outcome
what does Follow up over time mean
- Have participants changed exposure status over time?
- Has everyone been followed up over the entire study?
- How long do participants need to be followed up?
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 (bias if related to exposure and outcome)
- Potential for misclassification of exposures/outcomes
- Not good for rare outcomes
- Time consuming
- Expensive
Two types of cohort studies
- Prospective cohort studies
2. Historical cohort studies
Sometimes called retrospective cohort studies
difference between prospective and historical cohort study
starting points
Prospective - exposure
Historical - outcome
Prospective cohort studies
- May take long time
- Rare outcome
- Outcome takes long to develop
In a prospective cohort study, researchers
- Classifies exposure
- Follows by time
- Then observe the outcome
Historical cohort studies
- Quicker
- Less expensive than a prospective study
- Good for outcomes that take a long time to develop
Limitations
Use existing data (collected for other reasons) – quality?
- No control over quality of data
- Data may not include all the information that researchers would like to know
May not know about all relevant factors
Selection bias?
- Outcome occurred, the outcome influence exposure group
- Introduce selection bias
Strength of prospective
- Since outcome hasn’t happened
- When classifying people to whether or not they are exposed that won’t be affected by the outcome because it hasn’t happened yet
