Scatter Diagram is a graph that shows the relationship between two variables. It is one of the 7 basic quality tools used.
Scatter diagrams plot Ordered pairs (X,Y) on the X-Y axis. Scatter diagrams are also called “Correlation charts”, because they seek to explain a change in a dependent variable Y, in relation to a change observed in the corresponding independent variable X.
The direction of the correlation between the variables X and Y in a scatter diagram maybe 1) Positive (Proportional) or 2) Negative (Inverse) or 3) Zero (Not existent).
If a correlation can be established between the variables, then a regression line can be calculated and it can be used to estimate how a change to an independent variable will influence the value of the dependent variable.
A Scatter diagram can demonstrate a relationship between any element of a process, environment, or activity on one axis and a quality defect on the other axis.
Check the Seven Basic Quality Tools.