Epidemiology, Study Design, Bias and Confounding
Systematic and random error, internal and external validity, confounding, randomisation, concealment, blinding, causal reasoning and quantitative study designs.
How to prepare for MRCPsych Paper B
Integrate clinical formulation, safety and current UK practice with disciplined appraisal of design, bias, estimates and applicability.
Core concepts
Concept 1
Study design should match the clinical question: randomised trials test interventions, cohorts estimate incidence and prognosis, case-control studies efficiently examine rare outcomes, and cross-sectional studies describe a point in time.
Exam cue: Identify how participants entered the study, when exposure and outcome were measured, and whether allocation or follow-up could differ.
Concept 2
Internal validity is threatened by selection, information and performance biases, confounding and random error; design and analysis address different parts of that threat.
Exam cue: For an apparent effect, ask whether bias, chance or a third variable could produce it before accepting causation.
Risk pitfalls and guardrails
Confusing random sequence generation with allocation concealment or participant blinding.
Guardrail: Do not choose an option because it is merely familiar or associated; show why it best answers this lead-in.
Calling every baseline difference a confounder without its relationship to exposure and outcome.
Guardrail: Do not choose an option because it is merely familiar or associated; show why it best answers this lead-in.
Memory anchors
What is internal validity?
The extent to which the observed result is trustworthy for the participants studied rather than explained by systematic error.
What is confounding?
Distortion of an exposure-outcome relationship by a third factor associated with both and not on the causal pathway.
What does allocation concealment prevent?
Foreknowledge of the upcoming assignment when participants are enrolled, reducing selection bias.
What is selection bias?
Systematic difference arising from how participants enter, remain in or are analysed in the study.
Why does randomisation help?
It tends to balance known and unknown prognostic factors between groups and supports an unbiased comparison.
Checkpoint rule
Do the check-up only after you can summarize each concept in one sentence and identify one dangerous pitfall from memory.
Knowledge Check (after reading)
Short check-up to confirm understanding of this module.
Check-up Questions
Investigators randomly allocate patients with depression to two therapies and compare remission. What design is this?
Researchers enrol people after a first psychotic episode and follow them for five years to identify predictors of employment. What design is this?
Answer all questions to submit.
Next step personalized recommendations
Continue learning
Move forward only after this module is stable.
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