Case-Control Studies Flashcards
What is a case-control study?
The observational epidemiologic study of persons with the disease of interest and a suitable control group of persons without the disease
What is a retrospective study?
- Start by knowing who has the disease
- Work back to find who was exposed
What is a prospective retrospective study?
Case and controls in a case-control study can be accumulated prospectively
How to approach a case-control study?
Compare frequency of exposure among cases and controls
What is the problem with case-control studies?
Cannot calculate disease incidence rates because a case-control study does not follow a disease-free population over time
What does an odds ratio of more than 1 mean?
A positive association between the exposure and the disease, meaning the exposure might be a risk factor.
What does an odds ratio of less than 1 mean?
Suggests a potential protective
effect
What does an odds ratio of 1 mean?
There is no association between the exposure and the disease.
How to calculate an odds ratio?
Odds of exposure in cases / Odds of exposure in controls
What are the essential features of a case-control design?
- Directionality (outcome to exposure)
- Timing (retrospective for exposure)
- Rare or new disease
What are the design options for case-control studies?
- Choosing a hypothesis
- Choice of cases
- Choice of controls
- To match or not to match
- Sample size
What needs to be considered when choosing the cases for case-control studies?
- Definition of outcome
- Age range for inclusion
- Temporal boundaries
- Geographical boundaries of study
Where can cases be sourced from for case-controlled trials?
Population-based
Hospital-based
How are controls chosen for case-control studies?
Controls must be chosen to represent the population of individuals who would have been identified and included as cases had they developed the disease
What are the population control advantages?
- Exclusion criteria easy to apply
- Cases and controls from same study base
- Representative of whole study base
What are the population control disadvantages?
*If low participation rates, possible bias
*Inconvenience of finding controls
*Recall bias
*May not be motivated to take part
What are neighbourhood controls?
Controls selected from residences in same geographical areas as the cases
What are the population control advantages?
- Selection doesn’t need a roster of people
- Possible risk factors which vary geographically maybe more balanced between cases and controls
What are the population control disadvantages?
- Costly
- Possibly not representative of study base
- Households as sampling units
- Difficult to document non-response
- Possible over-matching if exposure is related to residence
What are hospitalised or disease registry controls?
- Control diseases should be subject to same surveillance and detection procedures as cases
- Controls come from same source as cases
What are the advantages of hospitalised controls?
- Easy to identify controls
- Readily available in sufficient numbers
- More likely to be aware of previous exposures or events than healthy individuals
- Likely to be subject to same selection factors determining hospitals attended
- More likely to cooperate than healthy individuals
What are the disadvantages of hospitalised controls?
- By definition, they are ill,
- Disease may have a common aetiology or be on the causal
pathway - BIAS
What are friend controls?
- Convenient & cheap
- Useful when friendship characteristics unrelated to exposure
- Over-matching when they are related to the exposure
How many controls are used in case-control studies?
One is standard but 1 can be a replication groups.
Control groups should be independent of each other
