Study Designs - Lecture Nineteen Flashcards
Cohort Studies
Cohort Studies step one
Identify a source population
Cohort Studies step two
Recruit your sample population
Cohort Studies step three
Assess exposure to identify which group participants belong to
Cohort Studies step four
Follow up overtime
Cohort Studies step five
Observe whether or not participants develop the outcome
Cohort Studies step six
Calculate measures of occurrence and measures of association
What can we measure using cohort studies?
Measures of occurrence - Incidence proportion - Incidence rate Measures of association - Relative risk - Risk difference
Incidence proportion
Number of people who develop the disease in a specific period / Number of people at risk of developing the disease at the start of the period
Incidence rate
Number of people who develop the disease in a specific period / Number of person-years at risk of developing the disease
Relative risk
How many times as likely it is that the exposed group will develop the disease compared to the comparison group
Rate ratio: IRe/IRc
Risk ratio: IPe/IPc
Risk difference
How many extra/fewer cases of the outcome there were in the exposed group compared to the comparison group
IRe-IRc
IPe-IPc
What might we need to consider carefully - Step One
- Identify a source population
Ideal: random selection independent of exposure status
Sometimes: Selection based on exposure status (might consider appropriate comparison group)
What might we need to consider carefully - Step Two
- Recruit your sample population
Can you be sure that the sample population does not already have the outcome?
What might we need to consider carefully - Step Three
- Assess exposure to identify which group participants belong in (exposed or not exposed i.e. comparison)
Have participants been correctly classified?
What might we need to consider carefully - Step Four
- Follow up over time
Have participants changed exposure status over time?
Has everyone been followed up over the entire study? Otherwise lost to follow up
How long do patients need to be followed up?
What might we need to consider carefully - Step Five
- Observe whether or not participants develop outcome
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
Potential for misclassification of exposures/outcomes
Generally not good for studying rare outcomes
Time consuming
Can be expensive
What could be done instead of cohort studies?
Case-control studies
Historical cohort studies
Types of cohort studies
Prospective cohort studies
Historical cohort studies
Prospective cohort studies
The research starts with classifying the exposure
Historical cohort studies
The exposure and outcome have already happened
Historical cohort studies strengths
Use existing data
Reconstruct follow-up period in the past
Less time consuming compared with prospective cohort studies
Less expensive
Good for outcomes the take a long time to develop
Historical cohort studies limitations
The quality of existing data
May not know all relevant factors
Selection bias
