GCSE Maths Scatter Graphs & Correlation Notes
A scatter graph plots pairs of values to show whether two variables are related. Correlation describes the strength and direction of that relationship.
Key concepts
- Positive correlation
- As one variable increases, so does the other — points slope upwards.
- Negative correlation
- As one variable increases, the other decreases — points slope downwards.
- No correlation
- No clear pattern — points are scattered randomly.
- Line of best fit
- A straight line through the middle of the points, used to make predictions.
Key formulas
Prediction
read from the line of best fit at the known value
Worked example
Points show that as revision hours rise, test scores rise. Describe the correlation and how to predict a score.
- 1The points slope upwards, so there is positive correlation.
- 2Draw a line of best fit through the middle of the points.
- 3To predict a score, go up from the revision hours to the line, then across to the score axis.
Answer: Positive correlation; read the prediction off the line of best fit
Common mistakes
✗ Assuming correlation means one thing causes the other.
✓ Correlation shows a link, not necessarily cause and effect.
✗ Drawing a line of best fit through the origin unnecessarily.
✓ The line should follow the trend of the points, not force through (0, 0).
✗ Predicting far outside the data range.
✓ Extrapolation beyond the data is unreliable.
Quick check
Try these, then reveal the answers.
Points sloping down show what correlation?Reveal answer
Negative.
What is a line of best fit used for?Reveal answer
Making predictions.
Random scatter shows what?Reveal answer
No correlation.
Exam tips
- Describe correlation in words: positive, negative or none, plus 'strong' or 'weak'.
- Draw one straight ruler line of best fit that balances the points either side.
- Note that predictions outside the data range are unreliable.
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Frequently asked questions
What does correlation tell you?+
The strength and direction of the relationship between two variables, but not that one causes the other.
How do I use a line of best fit?+
Read across and up from the axes to the line to estimate an unknown value.
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