This scatter graph sample illustrates correlation between price per 100 widgets in pound sterling and number of widgets purchased.
Correlation measures the strength of association between two variables. We will first consider the relationship between two scalar variables and then between ranked variables.
Pearson correlation coefficient.
Pearson's R indicates the strength and direction of association between two scalar variables, ranging from -1 which indicates a strong inverse relationship and 1 indicating a strong direct relationship. At 0 we say there is no correlation; it measures the linear dependence of one variable on another. Linear dependence means that one variable can be computed from the other by a linear equation ...
An inverse correlation.
In the following data we see that as the number of widgets rises, the price per 100 widgets falls. This is an inverse correlation and has a negative value for Pearson's R. ...
For this data the correlation coefficient has a value of -1." [en.wikibooks.org/wiki/Statistics_Ground_Zero/Association]
The scatter chart example "Widgets price" was created using the ConceptDraw PRO diagramming and vector drawing software extended with the Scatter Diagrams solution from the Graphs and Charts area of ConceptDraw Solution Park.