What is a Scatter Diagram?
A scatter diagram plots two variables against each other, one dot per observation, to reveal whether they move together. A rising cloud suggests positive correlation; falling, negative; a shapeless blob, none. It's the fastest visual test of "does X actually influence Y?"
The mandatory caution — correlation isn't causation — is itself exam material: the diagram raises hypotheses; experiments confirm them.
Worked example
A quality engineer suspects pour temperature affects weld-porosity rates. Sixty data points on a scatter plot show porosity climbing steeply above 32°C — a clear rising pattern. The DOE that follows confirms causation, and summer pours move to early morning. The scatter diagram didn't prove it; it told the team exactly what to test.