Randomised Trials & Fair Comparisons
Appraise treatment assignment, follow-up and analysis before trusting a trial’s conclusion.
Quick revision ↓ Medical Dictionary
01Generating and concealing assignment
A valid random sequence uses an unpredictable chance process rather than alternation, birth date or clinician choice. Allocation concealment prevents foreknowledge of the next assignment during recruitment. Without it, staff may consciously or unconsciously influence which people enter each group. These protections occur around assignment; blinding after assignment addresses different problems and does not repair a predictable or manipulated enrolment sequence.
02Blinding and outcome assessment
Masking participants, clinicians or outcome assessors can reduce behaviour changes and biased measurement. State who was blinded rather than relying on an ambiguous label such as double blind. Some interventions cannot be masked, but independent blinded assessment may remain possible. Objective outcomes can still be affected by differences in follow-up or co-interventions, so lack of masking requires a contextual judgement rather than an automatic verdict.
03Follow participants carefully
A participant flow diagram should account for screening, randomisation, treatment received and outcome availability. Examine reasons and timing of losses in each group. Equal dropout percentages do not guarantee unbiased results if the reasons differ. Document adherence, crossover and additional treatment. Adverse events need systematic collection rather than reporting only events volunteered by participants who remained enthusiastic about the intervention.
04Intention-to-treat interpretation
An intention-to-treat approach compares groups as originally assigned and targets the effect of assignment, preserving the benefits of randomisation as far as outcome data permit. Missing outcomes still require assumptions and sensitivity analysis. A per-protocol comparison addresses a different question and can lose randomisation's protection when adherence is related to prognosis. Read the actual methods rather than accepting an analysis label at face value.
05Read beyond the headline
Check the registered primary outcome, effect estimate, interval, harms and duration. Consider whether participants resemble those in your practice and whether the intervention can be delivered similarly. For cluster trials, analysis must account for group assignment. For crossover trials, carryover and period effects matter. A statistically favourable average result still requires judgement about individual benefit, treatment burden and alternative options.
High-Yield Revision
- Sequence generation, concealment and blinding are separate safeguards.
- Analyse the effect that matches the clinical question.
- Follow-up losses require reasons and sensitivity analysis.
Common Pitfalls
- Alternating treatment is predictable and is not true random allocation.
- An intention-to-treat label does not solve missing outcomes automatically.
References & Further Reading
Use the full methods and relevant chapters when appraising a paper. These sources support further study; the examples and questions above are original teaching exercises.
- Oxford CEBM — Critical appraisal tools
- Cochrane Handbook — Methods for evidence synthesis
- CDC — Analysing and interpreting epidemiological data
- ASA — Statement on p-values and statistical significance
- Oxford CEBM — Likelihood ratios
- Cochrane — GRADE and summary of findings
- WMA — Declaration of Helsinki, 2024 revision
- EQUATOR — Reporting guideline library
- NIH OBSSR — Mixed-methods research resources
- Institute for Healthcare Improvement — Model for Improvement