The Psychology of Quality and More

A Toolbook for Quality Improvement and Problem Solving (contents)

Scatter Diagram: Practical variations

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When to use it | How to understand it | Example | How to do it | Practical variations

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Practical variations

• If points on the Scatter Diagram coincide with other points, the fact that one point is actually two or more may be highlighted by emboldening them or by using concentric circles.
• If measurements are difficult to obtain, as few as 30 measurement pairs can be used.
• Use a Correlation Table when multiple coincidental points are measured, typically when there are a limited number of possible positions. This is effectively a cross between a Scatter Diagram and a Check Sheet, where each x-y position is represented by a box in which multiple points can be indicated.

Fig. 1. Correlation Table

• Separate measurement sets may be shown on the same Scatter Diagram, which may be distinguished from one another by using different shaped markers for each set of points. A typical use is where one variable is being changed, for example to show measurements of material from several different suppliers.
• Where non-linear correlation appears, rough estimates may be made using them by dividing them into approximately linear sections and calculating the regression line and standard error as above.

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