Scatter Diagrams
9.5 Scatter Diagrams – Cheat Sheet
1. Drawing a Scatter Diagram
- Plot data points on a coordinate grid using ordered pairs (x, y).
- Points should be marked clearly (×).
- Horizontal axis: independent variable (cause).
- Vertical axis: dependent variable (effect).
Example:
| Hours Studied (x) | Test Score (y) |
|---|---|
| 1 | 45 |
| 2 | 55 |
| 3 | 60 |
| 4 | 70 |
2. Types of Correlation
| Correlation Type | Description | Diagram Shape |
|---|---|---|
| Positive | As x increases, y increases | ↗ pattern |
| Negative | As x increases, y decreases | ↘ pattern |
| Zero | No visible relationship between x and y | scattered |
3. Line of Best Fit
- Drawn by inspection (not by calculation for O Level/IGCSE).
- Should extend across the whole range of the data.
- Equal spread of points above and below the line.
- Used to:
- Show trend
- Estimate values within the data range (interpolation)
- Predict values outside the data range (extrapolation – less reliable)
4. Interpreting Scatter Diagrams
- Look for the general trend (positive, negative, zero).
- Identify and comment on outliers (points far from the line).
- Strength of correlation:
- Strong = points close to the line
- Weak = points widely scattered around the line