The Scatter Diagram (also sometimes known as a Scattergram or Scatterplot) helps to identify the existence of a measurable relationship between two items by measuring them in pairs and plotting them on a graph, as below. This visually shows the correlation (but not the causal relationship) between the two sets of measurements.
When to use it
Use it when it is suspected that the variation of two items is connected in some way, to show any actual correlation between the two.
Use it when it is suspected that one item may be causing another, to build evidence for the connection between the two.
Use it only when both items being measured can be measured together, in pairs.

Fig. 1. Using the String Diagram in problem solving
How to understand it
When investigating problems, typically when searching for their causes, it may be suspected that two items are related in some way. For example, it may be suspected that the number of accidents at work is related to the amount of overtime that people are working.
The Scatter Diagram helps to identify the existence of a measurable relationship between two such items by measuring them in pairs and plotting them on a graph, as below. This visually shows the correlation between the two sets of measurements.

Fig. 1. Points on Scatter Diagram
If the points plotted on the Scatter Diagram are randomly scattered, with no discernible pattern, then this indicates that the two sets of measurements have no correlation and cannot be said to be related in any way. If, however, the points form a pattern of some kind, then this shows the type of relationship between the two measurement sets.
A Scatter Diagram shows correlation between two items for three reasons:
There is a cause and effect relationship between the two measured items, where one is causing the other (at least in part).
The two measured items are both caused by a third item. For example, a Scatter Diagram which shows a correlation between cracks and... [continues]

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