Scatter DiagramThis tool is very useful in finding the relationships between two variables. Some people do changes to the systems without analyzing the effect of it on other parts of the system. In such scenarios scatter diagram can be used to analyze these variations and find relationships if there are any. Even if you have not done a change to the system, this tool can be used in finding relationships between two variables. For an example x axis will have the lot size and y axis the amount of quality defects if we are trying to find a relationship between the lot size and quality defects. This will allow you to plot a diagram like below. 
This shows a positive correlation between the number of defects and lot size. With the lot size the amount of quality defects also goes up. But a graph like below does not show a positive correlation between the lot size and the number of defects.
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