A scatter graph is a graph used to show whether there is a relationship between two sets of data.
For example, you might investigate whether students who spend more time revising tend to get higher marks in a test.
Understanding scatter graphs
There are three main types of correlation you need to know.
Positive correlation
As one variable increases, the other tends to increase.
Example: More revision, higher test marks.
Negative correlation
As one variable increases, the other tends to decrease.
Example: Greater speed, less time to complete a journey of a fixed distance.
No correlation
There is no clear relationship between the two variables.
Example: Shoe size and test marks.
How to describe correlation in an exam
Use these three key phrases:
- Positive correlation
- Negative correlation
- No correlation
You might also need to describe the strength of the correlation:
- Strong correlation: the points are close to a straight line.
- Weak correlation: the points are more spread out.
For example, you could write: “There is a strong positive correlation.”
Remember: correlation does not necessarily mean that one thing causes the other.
Line of best fit
A line of best fit is a straight line drawn through the middle of the points on a scatter graph.
When drawing a line of best fit:
- Use a ruler.
- Draw a straight line through the middle of the points.
- Try to have roughly the same number of points above and below the line.
- Do not join the dots one by one.
Using a scatter graph to make estimates
You may be asked to estimate a value using your line of best fit.
- Interpolation: estimating a value within the range of the data you have.
- Extrapolation: estimating a value outside the range of the data you have. This is less reliable.
Remember these four things for your GCSE
- Recognise positive, negative and no correlation.
- Describe correlation as strong or weak.
- Know how to draw a line of best fit.
- Understand interpolation and extrapolation.