In one line: A correlational study measures two things in the same people and asks whether they go together. Nothing is changed.
You may have noticed that the friends who stay on their phone latest are the ones yawning in the first lesson.
You did not make anyone stay up. You only noticed that two things seem to go together: late screen time and tiredness.
That is the idea of a correlation.
How it works
- Choose two variables: hours of screen time after 9 pm, and hours of sleep.
- Measure both in the same people.
- See whether the two sets of numbers move together.
A correlational study measures its variables as they already are. There is no independent variable, because the researcher sets nothing.
It is used when a variable cannot, or should not, be changed on purpose. Nobody can ethically make teenagers lose sleep for a month.
Remember: Two measurements. No manipulation.
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Key idea: A correlation has a direction (positive or negative) and a strength (from none to perfect).
The direction
Positive correlation
Both variables rise together.
More hours of revision, higher marks.
Negative correlation
As one variable rises, the other falls.
More screen time at night, fewer hours of sleep.
No correlation
There is no pattern.
Shoe size and test marks.
Together · Opposite · No pattern
A graph like this is a scatter plot. The strength of the link is given by one number, the correlation coefficient, written r.
It runs from -1 to +1. The sign gives the direction. The size gives the strength: the closer to 1 or -1, the more tightly the dots sit around a line. An r of 0 means no relationship.
A common slip: r = -0.8 is a stronger correlation than r = +0.4.
The minus sign only tells you the direction.
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Key idea: Two things can go together without one causing the other.
Three other explanations
A third variable
Something that was not measured drives both.
Hot weather sends people to buy ice cream and to swim, so sales and drowning rise together.
The direction is unclear
Does late screen time cut sleep? Or do teenagers who cannot sleep pick up their phone?
When we cannot tell which way the effect runs, it is called bidirectional ambiguity.
Chance
Some things rise together by coincidence.
A link that means nothing is called a spurious correlation.
Something else · The other way round · Chance
There is one more limit. A correlation only describes the data that were collected.
If students who revise for one hour score 5 out of 10, it does not follow that two hours will give 10 out of 10.
Exam tip: When a claim says "X causes Y" and the study was correlational, ask three things.
Could something else cause both? Could it run the other way? Could it be chance?
Key idea: A correlational study can go where an experiment cannot. What it cannot do is say why.
Strengths
- Studies what would be unethical to change: smoking, stress
- Shows whether two variables are related, and how strongly
- Can use data that already exist: quick and cheap
- Finds a pattern that an experiment can then test
Limits
- Nothing is changed, so no cause and effect
- A third variable may explain the link
- The direction may be the other way round
- Easily read as a cause, especially in headlines
Correlational studies and experiments often work as a pair.
The correlation spots a pattern in real life: people under long-term stress are ill more often. An experiment, where one is ethically possible, then tests whether the link is causal.
Choose your words: For a correlation write is linked to, is related to or goes with.
Keep causes, leads to and makes for experiments.
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How this is asked: A correlational study is not one of the class practicals, so you are asked to compare it with the method your class did use, for 6 marks.
And whenever a study you are given is correlational, its result must not be read as a cause.
Your class practicals
- Your class runs four small studies of its own, called class practicals.
- They are an experiment, an interview, an observation and a survey.
- Paper 2 asks about the ones your class ran.
- Keep a short note of each: aim, what you did, who took part, what you found, and the ethics.
The answer pattern
- Two or three similarities, each starting with "Both".
- Two or three differences, each using "whereas".
- Your own practical in every paragraph.
The trap: the word "causes": "The correlational study shows that stress causes illness" is wrong in any answer.
A correlation shows that two things are related.
Your class practical was an experiment. Your class tested whether students solve more sums in silence than with the radio on. Compare and contrast the research methodology of the experiment used in your class practical with the research methodology of a correlational study.
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