Experiments: designs, groups and controls
Compare repeated and independent measures, apply counterbalancing and random allocation, and control extraneous variables.
Original teaching mapped to Cambridge IGCSE Psychology 0266 Version 3 for 2027–2029. The course is independent study support, not Cambridge material, an official mark scheme or an awarded grade. Named-study notes summarise the required findings; candidates are not expected to evaluate named studies. Self-check practice is not official grading, a timed Paper 1/2 simulator or teacher review.
Before you begin
- Use the Version 3 syllabus and confirm availability in your administrative zone with Cambridge and your school. Both papers are compulsory. The updated named-study summaries are required context; full original research articles are not required.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Before experiments: designs, groups and controls, which approach is most useful?
What you will learn
- Define experimental design and distinguish repeated and independent measures
- Explain counterbalancing and random allocation
- Distinguish conditions, experimental groups and control groups
- Identify extraneous variables and propose controls
Design determines who experiences each condition
An experimental design is how participants are assigned to the IV conditions. In repeated measures, the same participants experience every condition. Counterbalancing changes the order to reduce order effects. In independent measures, different participants experience each condition; random allocation gives each person an equal chance of entering any condition.
A control group gives a comparison
An experimental condition is an IV level. An experimental group experiences the intervention; a control group does not receive the experimental condition and provides a comparison. State exactly what each group does rather than assuming ‘control’ means no activity.
Control other influences on the DV
An extraneous variable is any variable besides the IV that could affect the DV. A control is a step used to reduce or hold its impact constant, such as using the same instructions, time of day, materials and scoring rule. Syllabus boundary: matched-pairs and mixed designs are not required; do not present them as required answers.
Pause and explain
Which response best shows understanding of experiments: designs, groups and controls?
Worked example
The same pupils complete a recall test with and without background music. What design is this, and how could order be controlled?
Show the worked solution
- The same participants do both conditions.
- That is repeated measures.
- Counterbalance which condition comes first.
Answer Repeated measures; counterbalance the order to reduce practice or fatigue effects.
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.
The same participants complete a test in silence and with music. Name the design and one likely order control.
Give me a hint
Track who takes part in each condition.
Compare my reasoning
- The same people experience both levels.
- This is repeated measures.
- Counterbalance the order to reduce practice or fatigue effects.
Repeated measures; counterbalance condition order.
Look for these in your work
- Identifies same participants
- Names the design
- Gives an appropriate control
Different participants take either the control or experimental condition. How can allocation reduce pre-existing group differences?
Give me a hint
Every participant should have an equal chance at either level.
Compare my reasoning
- List eligible participants before assigning conditions.
- Use random allocation to assign each person.
- This reduces systematic selection differences between groups.
Random allocation gives each participant an equal chance of entering either group.
Look for these in your work
- Names random allocation
- Explains equal chance
- Links it to group comparability
A memory study tests music condition but lets one group have twice as long to learn the list. Identify the problem and propose a control.
Give me a hint
The groups differ on a factor besides the IV.
Compare my reasoning
- Study time can affect recall.
- It is an extraneous variable because duration differs between conditions.
- Use the same learning time for both groups.
Learning time is an extraneous variable; keep it constant across conditions.
Look for these in your work
- Identifies the variable
- Explains its effect
- Proposes a direct control
Common mistakes
- Calling random allocation the same as random sampling.
- Claiming a control group experiences no procedure at all.
- Introducing matched-pairs or mixed designs as syllabus knowledge.
What can you explain now?
Which design uses the same participants in every IV condition?
Compare with the explanation
Repeated measures
Independent measures assigns different people to different conditions.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
How do participant assignment, control groups, counterbalancing and control of extraneous variables strengthen an experiment?
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.