Key Idea: Two things that go together are not yet cause and effect. A cause is shown by changing one thing and ruling out every other explanation.
A link is not a cause
- Correlation: two things change together
- A third variable may cause both
- The arrow may point the other way
How to show a cause
- Change one thing, keep the rest the same
- Random allocation: groups alike at the start
- Placebo and blinding: against expectation
Checking the claim
- Internal validity: was it really the IV?
- Significance: is it more than chance?
- External validity: does it hold elsewhere?
When it is not simple
- Bidirectional: each affects the other
- Interaction: an effect depends on another factor
- One simple cause is rarely the whole story
The words to know
| Word | What it means |
|---|---|
| Correlation | Two things tend to change together |
| Causation | One thing makes another happen |
| Third variable | A hidden factor that causes both of the things measured |
| Reverse causation | The effect is really the cause |
| Independent variable | The one thing the researcher changes on purpose |
| Random allocation | Chance decides which group each person joins |
| Placebo | A fake treatment that looks like the real one |
| Internal validity | Confidence that the IV, and nothing else, caused the change |
| Statistically significant | Unlikely to be due to chance alone |
| Bidirectional relationship | Each of two things affects the other |
Was anything changed, or only measured? What else could explain it, and which way does the arrow point?
Exam-style question
Your class practical was a survey. Your class asked 45 students how many hours a week they play video games, and measured each student's reaction time with a ruler-drop test. Students who played more had faster reaction times. Explain the concept of causality in relation to the survey in your class practical.
Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
Important: "Causes", "improves" and "leads to" need an experiment with a fair comparison. For anything else, write "is linked to".
Tap each card to check yourself.
Ice-cream sales and drownings rise together. Why? A third variable: hot weather drives both.
What are the two variables in an experiment called? The independent variable (changed) and the dependent variable (measured).
Why allocate people to groups at random? So the groups are alike at the start and only the IV differs.
What does a placebo control for? Expectation: both groups believe they may be treated.
"Statistically significant" means what? The difference is unlikely to be due to chance alone.
Stress and sleep each make the other worse. What is this? A bidirectional relationship, a feedback loop.
Exam tips
- Ask first: was a variable changed, or only measured?
- Give a third variable or a reversed direction that fits this study.
- For an experiment, check how people got into the groups.
- Keep "significant" and "caused by the IV" apart.