In one line: A correlation means two things change together. That is all it says.
In summer, ice-cream sales go up. So do sunglasses sales. In winter, both fall.
They rise and fall together. That is a correlation.
Notice what it does not say: buying an ice cream does not make anyone buy sunglasses. The two just travel together.
Coffee and alertness
People who drink more coffee are often more alert.
The pattern alone does not say why. Perhaps people who need to stay alert reach for coffee.
Exercise and happiness
People who exercise more tend to be happier.
They move together, but which one leads to the other? The correlation cannot say.
Shoe size and vocabulary
Children with bigger feet know more words.
Older children have both.
Remember: Correlation = they move together.
It describes a pattern. It does not say why.
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In one line: Causation means one thing makes the other happen.
Stay in the summer, but look at a different pair: hot weather and ice-cream sales.
When a heatwave arrives, sales jump. When it cools down, they fall. The heat makes people buy ice cream.
That is causation: there is an arrow from one thing to the other.
Rain and car accidents
Rain makes the roads slippery, so accidents rise. In a dry week there are fewer.
The rain changes the driving conditions: a cause.
Smoking and lung cancer
Smoking damages the cells that line the lungs, so the risk of cancer rises.
When people stop, the risk falls.
Sun and sunburn
Stay out unprotected and the sun burns your skin.
Block it with sunscreen and there is no burn.
The quick test: Change one thing. Must the other change?
Cool the weather and ice-cream sales fall: causation.
Ban ice cream and sunglasses sales do not care: only a correlation.
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Key idea: In the numbers, a correlation looks exactly like causation.
In summer, ice-cream sales rise, and so do drowning accidents. It is a strong correlation.
Does ice cream drown people? No. Something hidden drives both.
Three reasons a correlation is not a cause
A third variable
Something else causes both.
Hot weather sells ice cream and sends people swimming.
Reverse causation
The arrow may point the other way: B causes A.
An ice-cream van is always where the crowd is. Did the van bring the crowd, or the crowd bring the van?
Coincidence
Sometimes two things line up by luck.
Compare enough pairs of things and a few will match by accident.
Third variable · Reverse · Coincidence
A real study: the marshmallow test: Four-year-olds were offered one marshmallow now, or two if they could wait. Years later, the children who had waited longer were doing better at school.
Does self-control cause success?
A larger study in 2018 found that most of the link disappeared once the family's income and education were taken into account.
Family background was a third variable: it shaped both the waiting and the school results.
Take a real headline: "Teens who use social media more are more anxious."
It is tempting to say that social media causes anxiety. But anxious teens might use it more to cope, which is reverse causation. Or loneliness might drive both, which is a third variable.
The correlation alone cannot tell us which story is true.
What can show a cause: An experiment changes one thing on purpose and keeps everything else the same.
A study that only measures two things can flag a link worth testing. It cannot show the cause.
How this is asked: Causality is one of the four concepts named in the 15-mark question on a study you are given.
If the study only measured two things, it cannot show which one causes the other.
The pattern
- Say what kind of study it is: did anyone change anything, or only measure?
- Give a third variable that could explain the link in this study.
- Turn the arrow round: could the effect be the cause?
- Bring in a second concept, such as measurement or bias.
- Conclude: what can the study claim, and what would test the cause?
The trap: "third variable" with no example: Naming the idea is not enough.
Say what the third variable could be and how it would produce both results.
Discuss the following study with reference to two or more of the following concepts: bias, causality, measurement and/or responsibility.
Researchers asked 800 students aged 14 to 16, in an online questionnaire, how many days a week they eat breakfast and what grades they got in their last school report. Students who ate breakfast on more days reported higher grades. The researchers concluded that eating breakfast improves school performance, and a newspaper reported: "Breakfast boosts grades."
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