2.4: Study design in epidemiological research

Question 1

Fundamental principles of epidemiological study design (x2):

Answer 1

• Study should be comparative
• Study should seek to avoid bias

Question 2

Broad classes of epidemiological study design:

Answer 2

• Observational; role of investigator is passive, exposure are not manipulated
• Intervention; investigators play an active role in exposing groups to interventions of interest such as treatments -> clinical trials

Question 3

Benefit and drawback of intervention studies:

Answer 3

• Considered the gold standard in terms of causality
• Major ethics issue (forcing exposures on people)

Question 4

Give the 3 key designs for observational studies:

Answer 4

• Cohort studies: Exposure -> outcome
• Case-control studies: Outcome -> exposure
• Cross-sectional studies such as surveys -> only useful for prevalence

Question 5

Cohort studies: Principle and procedure

Answer 5

• Tracking two or more groups forward from exposure to outcome
• Studying occurrence of disease after exposure at a specified point in time -> compare exposure groups with respect to disease outcomes
• Most useful type of observational study for determining disease aetiology
• Providing strongest evidence for causality since exposure is known to precede disease

Question 6

Drawbacks of cohort studies:

Answer 6

• Large and costly
• Selection bias (observational study, prognostic factors)
• Not useful for rare diseases (long time to develop)
• Loss to follow-up
• Exposures can change during study period
• Confounding variables

Question 7

Case-control studies: Principle and procedure

Answer 7

• Case and control groups are defined and selected according to disease status
• Look back at their histories to ascertain exposure status

Question 8

2 advantages and 2 disadvantages of case-control studies:

Answer 8

+

• Efficient (time and money)
• Can study rare disease

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• Choosing control group exerts control bias
• Obtaining exposure history exerts recall bias