Definition
An exploratory chart that plots two paired variables as coordinate points to observe whether there is a directional, strength, or nonlinear relationship.
In detail
Scatter diagrams are used to propose and screen hypotheses about variable relationships and are one of the seven quality tools. Each point must come from a pair of data from the same object or the same time; the horizontal axis usually holds the possible cause and the vertical axis the result. Graphical correlation does not equal causation, and abnormal points cannot be deleted without factual basis; if there are time trends, grouping confounding, or common causes, further stratification, experiments, or process knowledge are needed for verification.
Example in practice
An engineer suspected that oven temperature affected bonding strength, so he plotted a scatter diagram of actual temperature and corresponding strength for each batch. The point cloud showed that strength first increased then decreased as temperature rose, suggesting the relationship might not be simply linear. The team then designed a controlled experiment to confirm the optimal window, rather than directly modifying the standard based only on the correlation chart.