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A Level · Cambridge (CIE) · 9990

Evaluate randomised control trials

Understand how random allocation and a comparison condition help test an intervention, and where a trial can still mislead.

Section AL-RCT · Randomised control trials

Cambridge International AS & A Level Psychology 9990 · 2028–2030 · Version 1 · AL-RCT
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?

Put the idea to work

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
  1. Identify how students are assigned and what the two conditions are.
  2. Name one outcome measured consistently in both groups.
  3. 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.

1 · Guided

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
  1. Chance assignment to the two conditions is random allocation; it is not random sampling.
  2. The wellbeing course is the active comparison condition.
  3. 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
2 · Independent

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
  1. The groups have substantially different follow-up rates.
  2. If those who leave differ in wellbeing or response to the course, the remaining scores may not represent the original groups equally.
  3. 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
3 · Transfer

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
  1. Recruit students with appropriate permission, then assign by chance to reminders or the usual routine while keeping required teaching available to everyone.
  2. Define an outcome such as completed practice sessions in a four-week period and record it the same way for both conditions.
  3. 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
Recall without your notes

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.

After trying it yourself, choose your next review. This is your self-assessment.

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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.

Keep learning Psychology

Course scope and assessment

Seven original lessons map all 15 AS research-method and methodological-concept sections to explained Guided, Independent and Transfer practice. All 12 compulsory AS core studies have original study-specific teaching and staged practice; five additional A Level methodology lessons cover research design and validity. All 15 Clinical, all 15 Consumer (2.1.1–2.5.3) and all 15 Health (3.1.1–3.5.3) Psychology sections link to original teaching and staged practice. All 15 Organisational Psychology sections 4.1–4.5 now have original teaching and explained staged practice; the six new 4.4–4.5 lessons bring the planning map to 92 of 92 indexed sections. Consumer lessons cover physical and psychological environments; decision models, heuristics and mistakes; packaging, selling and buying; advertising, self-monitoring, product placement, brand recognition and slogans. Evaluation boundaries are explicit: studies use particular products, samples, tasks and settings; gaze, recall, self-report, EEG indices, ratings, choices and purchases are different outcomes. Health evidence is bounded: Laba et al. measured hypothetical preferences; Riekert and Drotar raise a non-response concern; Chung and Naya's cap tracked openings, not swallowing; Chaney et al. was a short caregiver-reported pilot; Yokley and Glenwick examined immunisation attendance in one public-health setting. MacLachlan et al. report one patient, not a controlled treatment effect; Brudvik et al. compared separate child, parent and physician ratings in one emergency department. Chandola et al. found a cohort association; Wang et al. measured responses during a monitored arithmetic task; Evans and Wener observed commuters; Bridge et al. found mixed self-rated outcomes in radiotherapy. Janis and Feshbach measured short-term self-reports in one school; Lewin et al. tested a rehabilitation package, not information alone. Tapper et al. and Fox et al. studied bounded school and worksite programmes; incentive effects do not establish transfer or rule out reporting effects. Weinstein measured comparative expectations, while Seligman’s framework is conceptual and Shoshani and Steinmetz used a school-level comparison. These findings do not establish universal health effects or diagnose individuals. Need-theory examples use a child measurement study, not a workplace test; SMART is a practical checklist, not a motivational theory. Landry et al. compared reward framing with student volunteers in a lab; this does not establish universal effects in workplaces. Cuadrado et al. (2008) measured undergraduate evaluations of presented leaders, not workplace performance or hiring; this does not establish a universal gender effect. The historical 1995 Munchausen criteria are not a current diagnostic checklist. Cambridge’s 2028–2030 Reference List treats Oruč et al. (1997) as an example of genetic explanation although its participants had bipolar I; gambling disorder is listed in Cambridge’s Clinical topic but classified under addictive behaviours in ICD-11. Candidates choose two options, but all four remain listed so schools can select a route. The 92-section count is a planning index, not a Cambridge score. Remaining work is deeper coverage review, separate-year syllabus reconciliation, timed-paper practice and human subject-expert review. The separate 2027 and 2024–2026 syllabuses are not reconciled into this route. Practice is not official grading, a timed-paper simulation or human subject-expert review.

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