In one line: To prove cause, change one thing on purpose and hold everything else still.
Say we want to know if background music harms revision. In an experiment we deliberately change one thing — the independent variable (music on vs off) — and measure the dependent variable (test score).
If everything else is kept the same for both groups, then any difference in scores can be pinned on the music. That is how an experiment does what a correlation cannot: it shows cause.
Memory hook: Change the IV, measure the DV, freeze the rest. That's the recipe for cause.
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Key idea: Three tools rule out other explanations, so the IV is the only thing that could have caused the difference.
What makes an experiment causal
A control group
One group gets the IV (music), one doesn't. The comparison shows what the music added.
Random allocation
People are put in groups by chance, so the groups start out similar — no third variable sneaks in.
Control extraneous variables
Keep other things equal — same test, room, time — so only the IV differs.
Control group · Random allocation · Control variables
When all three hold, the study has high internal validity. That is exactly what lets us claim cause.
Go further — higher-level insight: Random allocation is the quiet hero. It's what balances out third variables you never even thought of — motivation, sleep, mood — spreading them evenly across groups so they can't explain the result.
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How this is tested: Causality is a Paper 2 Section B concept, and 'how would you show cause / design an experiment' is core Paper 2 research-methods skill. The move: turn a weak correlational claim into a proper experiment, and explain why that lets you claim cause.
A survey found that students who use flashcards get better grades. Explain how a researcher could design an experiment to test whether flashcards actually cause better grades.
Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
Common mistakes: 1. No random allocation. Without it, groups may differ to start.
2. Forgetting to control variables. Then something else could cause the result.
3. Not saying WHY it shows cause. Link back to internal validity.