Case-Control Studies

Question 1

What is a case-control study?

Answer 1

The observational epidemiologic study of persons with the disease of interest and a suitable control group of persons without the disease

Question 2

What is a retrospective study?

Answer 2

• Start by knowing who has the disease
• Work back to find who was exposed

Question 3

What is a prospective retrospective study?

Answer 3

Case and controls in a case-control study can be accumulated prospectively

Question 4

How to approach a case-control study?

Answer 4

Compare frequency of exposure among cases and controls

Question 5

What is the problem with case-control studies?

Answer 5

Cannot calculate disease incidence rates because a case-control study does not follow a disease-free population over time

Question 6

What does an odds ratio of more than 1 mean?

Answer 6

A positive association between the exposure and the disease, meaning the exposure might be a risk factor.

Question 7

What does an odds ratio of less than 1 mean?

Answer 7

Suggests a potential protective
effect

Question 8

What does an odds ratio of 1 mean?

Answer 8

There is no association between the exposure and the disease.

Question 9

How to calculate an odds ratio?

Answer 9

Odds of exposure in cases / Odds of exposure in controls

Question 10

What are the essential features of a case-control design?

Answer 10

• Directionality (outcome to exposure)
• Timing (retrospective for exposure)
• Rare or new disease

Question 11

What are the design options for case-control studies?

Answer 11

• Choosing a hypothesis
• Choice of cases
• Choice of controls
• To match or not to match
• Sample size

Question 12

What needs to be considered when choosing the cases for case-control studies?

Answer 12

• Definition of outcome
• Age range for inclusion
• Temporal boundaries
• Geographical boundaries of study

Question 13

Where can cases be sourced from for case-controlled trials?

Answer 13

Population-based
Hospital-based

Question 14

How are controls chosen for case-control studies?

Answer 14

Controls must be chosen to represent the population of individuals who would have been identified and included as cases had they developed the disease

Question 15

What are the population control advantages?

Answer 15

• Exclusion criteria easy to apply
• Cases and controls from same study base
• Representative of whole study base

Question 16

What are the population control disadvantages?

Answer 16

*If low participation rates, possible bias
*Inconvenience of finding controls
*Recall bias
*May not be motivated to take part

Question 17

What are neighbourhood controls?

Answer 17

Controls selected from residences in same geographical areas as the cases

Question 18

What are the population control advantages?

Answer 18

• Selection doesn’t need a roster of people
• Possible risk factors which vary geographically maybe more balanced between cases and controls

Question 19

What are the population control disadvantages?

Answer 19

• 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

Question 20

What are hospitalised or disease registry controls?

Answer 20

• Control diseases should be subject to same surveillance and detection procedures as cases
• Controls come from same source as cases

Question 21

What are the advantages of hospitalised controls?

Answer 21

• 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

Question 22

What are the disadvantages of hospitalised controls?

Answer 22

• By definition, they are ill,
• Disease may have a common aetiology or be on the causal
  pathway
• BIAS

Question 23

What are friend controls?

Answer 23

• Convenient & cheap
• Useful when friendship characteristics unrelated to exposure
• Over-matching when they are related to the exposure

Question 24

How many controls are used in case-control studies?

Answer 24

One is standard but 1 can be a replication groups.

Control groups should be independent of each other

Question 25

What is matching?

Answer 25

Matching characteristics to control for confounding factor.

Question 26

What are the disadvantages of matching?

Answer 26

* Increased effort to find suitable controls * Exclusion of cases if no match found * Longer study duration * OVERMATCHING

Question 27

What is overmatching?

Answer 27

A matching variable is involved in or is closely related to the mechanism whereby the exposure influences disease risk

Question 28

What is nested case-controlled study?

Answer 28

* Cases and controls drawn from a cohort study * Particularly useful when wanting more detailed measurements on a subset of the cohort

Question 29

What are the advantages of case-control studies?

Answer 29

* Several aetiologic factors for a single disease * Is optimal for the study of rare disease * Well suited for disease with long latency periods * Relatively quick * Yields estimates of relative risk

Question 30

What are the disadvantages of case-control studies?

Answer 30

* Potential for selection bias * Retrospective collection of information * Can only look at one disease * Yields only relative risk