Introduction to Scatter Diagrams
When we collect data on two variables for the same subjects — for example, the number of hours a student studies and their exam score — we call this <strong>bivariate data</strong>. A scatter diagram (or scatter plot) is the go-to tool for visualising such data and spotting any relationship between the two variables.
A graphical representation of bivariate data in which each data pair is plotted as a point on a coordinate plane. The horizontal axis represents one variable and the vertical axis represents the other.
To construct a scatter diagram:
- Decide which variable goes on each axis. The independent variable (the one you control or use to predict) typically goes on the -axis, and the dependent variable goes on the -axis.
- Choose appropriate, evenly-spaced scales for both axes.
- Plot each data pair as a single point.
- Label both axes with the variable name and units.
A researcher records the number of hours 8 students spent studying and their test scores (out of 100):
| Hours studied () | Test score () |
|---|---|
| 1 | 45 |
| 2 | 52 |
| 3 | 58 |
| 4 | 65 |
| 5 | 70 |
| 6 | 74 |
| 7 | 80 |
| 8 | 88 |
Each row becomes one point. The point means: studied 1 hour, scored 45. Plotting all eight points gives a clear upward pattern.
Always label your axes and include units — in IB exams, missing labels can cost you marks.