A scatter diagram shows the correlation between two variables in a process. These variables could be a Critical To Quality (CTQ) characteristic and a factor affecting it, two factors affecting a CTQ or two related quality characteristics. Dots representing data points are scattered on the diagram. The extent to which the dots cluster together in a line across the diagram shows the strength with which the two factors are related.
Sometimes the scatter plot may show little correlation when all the data are considered at once. Stratifying the data, that is, breaking it into two or more groups based on some difference such as the equipment used, the time of day, some variation in materials or differences in the people involved may show surprising results.
The reverse can also be true. A significant correlation could appear to exist when all the data are considered at once, but stratification could show that there is no correlation. One way to look for stratification effects is to make your dots in different colors or use different symbols when you suspect that there may be differences in stratified data.
Why Use Scatter Diagrams?
To study and identify the possible relationship between the changes observed in two different sets of variables. Scatter diagrams are especially useful in the measure and analyze phases of the Lean Six Sigma methodology.
What Does Scatter Diagrams Do?
- Supplies the data to confirm a hypothesis that two variables are related.
- Provides both a visual and statistical means to test the strength of a potential relationship
- Provides a good follow-up to a Cause & Effect Diagram to find out if there is more than just a consensus connection between the causes and the effect. You must take care, however, so that you do not misinterpret what your diagram means.
What Scatter Diagrams Can Do For You?
To control variation in any process, it is absolutely essential that you understand which causes are generating which effects. A cause and effect diagram can help you identify probable causes. Scatter diagrams can help you test them. By knowing which elements of your process are related and how they are related, you will know what to control or what to vary to affect a quality characteristic.
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