Evaluate randomised control trials
Understand how random allocation and a comparison condition help test an intervention, and where a trial can still mislead.
Section Paper 4 · Specialist Options: Application and Research Methods
Exam scope and assessment notes
Original StudyForA teaching mapped to the Cambridge 2028–2030 syllabus. These lessons are independent study support, not Cambridge material, official mark schemes or awarded grades. No lesson reproduces a research article or Cambridge question. Human subject-expert review and full timed-paper grading remain pending. This is one of the five additional A Level methodology areas and supports Paper 4 planning; it does not replace teaching the selected specialist options or a timed Paper 4 practice set.
Before you begin
- This lesson supports the 2028–2030 Version 1 A Level Psychology route. Review the AS research-method foundations first, then apply the method to the specialist options your school selects.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which feature makes an allocation randomised?
What you will learn
- Explain random allocation, intervention and comparison conditions, outcome measures and blinding where feasible
- Evaluate causal inference, group differences, attrition, generalisation and ethical limits in a trial
A trial compares an intervention with a planned comparison
A randomised control trial (RCT) assigns participants by chance to an intervention condition and a comparison condition. The comparison may be usual care, a placebo or another intervention; it must fit the question and must not withhold essential support.
Researchers define the outcome before the trial, apply the planned conditions and measure the groups in a consistent way. Random allocation is intended to distribute participant differences across conditions on average. It is not random sampling from the population and it cannot guarantee equal groups in a small trial.
Blinding and follow-up can protect the comparison
Where feasible, participants, practitioners or outcome assessors may be kept unaware of which condition was assigned. Blinding can reduce expectations affecting behaviour or scoring, but treatment effects may reveal an assignment and some interventions cannot be concealed.
Record who starts, completes and withdraws, when outcomes are measured, and whether the same protocol is followed. Unequal attrition, non-adherence, a weak comparison or selective outcome reporting can distort the apparent difference. A trial result supports the tested intervention, sample, measure and time period; it does not automatically generalise to every person or setting.
Causal evidence still has boundaries
Random allocation strengthens a causal comparison because assignment is not deliberately based on participants' starting characteristics. It does not remove measurement error, poor implementation, chance differences, co-interventions or loss to follow-up.
Ethical review, valid consent, proportionate risk, privacy and a safe stopping plan matter. Researchers should not deny effective support merely to create a comparison. In applied psychology, a carefully designed trial can improve evidence, but it is not by itself proof that an intervention is suitable for an individual.
Sources for this lesson
Pause and explain
Why might a trial use blinded outcome assessors?
Worked example
A school tests a new revision-planning programme against its usual study-support sessions. What makes this a randomised control trial, and what should be checked before interpreting the result?
Show the worked solution
- Identify how students are assigned and what the two conditions are.
- Name one outcome measured consistently in both groups.
- Check attrition, implementation and whether students needing support still receive it.
Answer It is an RCT if eligible students are assigned by chance to the new programme or usual support and the same pre-specified outcome is measured in both groups. Before interpreting a difference, check that both conditions were delivered as intended, that outcome scoring was consistent, that withdrawals did not differ in a way that biases the result, and that the comparison remained ethical.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A trial assigns consenting adults by chance to a brief mindfulness course or a time-matched wellbeing course. Name the allocation, comparison and one outcome that could be measured in both groups.
Give me a hint
Separate how people are assigned from how they were recruited.
Compare my reasoning
- Chance assignment to the two conditions is random allocation; it is not random sampling.
- The wellbeing course is the active comparison condition.
- A pre-specified measure, such as the same validated self-report wellbeing scale at baseline and after the course, can be collected in both groups.
This is random allocation to a mindfulness course or an active comparison. Measure the same defined wellbeing outcome at the same times in both groups, and describe how the measure is scored.
Look for these in your work
- Allocation method correctly named
- Comparison condition identified
- Outcome uses a consistent measure and timing
The mindfulness group improves more, but 30% of the comparison group and 8% of the mindfulness group do not complete follow-up. Explain one threat to interpretation.
Give me a hint
Ask whether missing outcomes may be related to condition or progress.
Compare my reasoning
- The groups have substantially different follow-up rates.
- If those who leave differ in wellbeing or response to the course, the remaining scores may not represent the original groups equally.
- Report attrition by condition and examine the missing-data pattern before making a causal claim.
Unequal attrition may bias the group comparison if completion is related to outcome. The result should be qualified and the missing-data pattern examined; the initial random allocation does not repair a biased set of completed scores.
Look for these in your work
- Unequal attrition identified
- A plausible source of bias explained
- Conclusion appropriately qualified
Plan a safe trial of a reminder routine that may help students practise vocabulary regularly.
Give me a hint
Do not remove normal teaching or make exam success the only outcome.
Compare my reasoning
- Recruit students with appropriate permission, then assign by chance to reminders or the usual routine while keeping required teaching available to everyone.
- Define an outcome such as completed practice sessions in a four-week period and record it the same way for both conditions.
- Set a privacy-conscious reminder schedule, allow students to stop, monitor withdrawals and check whether the routine works outside the trial period.
A small ethical trial can compare reminders with usual practice without withholding instruction. Predefine a practice-frequency outcome, use the same recording rule in both conditions, protect student data and review attrition before interpreting the difference.
Look for these in your work
- Conditions and chance allocation specified
- Observable outcome operationalised
- Support, privacy and stopping safeguards included
Common mistakes
- Confusing random allocation with random sampling
- Assuming randomisation guarantees identical groups
- Treating any no-treatment condition as an acceptable control
- Claiming a trial result proves effectiveness for every learner
- Ignoring attrition or a measure chosen after seeing the results
What can you explain now?
What does random allocation do in an RCT?
Compare with the explanation
It uses chance to assign participants to conditions, helping distribute participant differences across groups on average.
It does not select a representative sample or ensure perfect group equality.
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Make it stick
How do random allocation, a suitable comparison and consistent outcome measurement strengthen an RCT, and what important limits remain?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.