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By the end of these notes, you will be able to:
A scatter diagram (also called a scatter graph or scatter plot) is a type of graph that shows whether two things are related to each other. You plot pairs of data — one value on the horizontal axis (x-axis) and the other on the vertical axis (y-axis) — and look at the overall pattern of the points.
Example: You might want to know — do students who study more hours get higher test scores? You would collect data from several students (hours studied + their score), then plot each student as a single point on the graph.
Key plotting rule: Every point must be marked as a small cross (×), not a dot, blob, or circle. This makes each point clear and precise.
Step 1: Draw your axes. Label the horizontal axis (x-axis) with one variable and the vertical axis (y-axis) with the other. Include units if there are any (e.g., hours, °C, £).
Step 2: Choose a suitable scale for each axis. Make sure your scale fits all the data values and spreads the points out clearly across the graph.
Step 3: Plot each pair of values as a cross (×). For each data pair, find the x-value along the horizontal axis and the y-value along the vertical axis, then mark a cross where the two meet.
Step 4: Do not join the crosses with lines. The points stay separate — this is what makes it a scatter diagram, not a line graph.
Example: The table below shows the price of 5 computers and the time (in seconds) each takes to run a program:
| Price (£) | Time (seconds) |
|---|---|
| 250 | 5.1 |
| 320 | 3.2 |
| 400 | 4.1 |
| 650 | 2.8 |
| 900 | 2.6 |
You would plot the point (250, 5.1), then (320, 3.2), and so on — each as a cross on the graph.
Correlation is the word we use to describe the relationship (or connection) between two variables on a scatter diagram. Looking at the overall shape or direction of the plotted crosses tells you what type of correlation exists.
There are three types:
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