We now consider a different type of data in which two different measurements are made from each of several groups. These groups are often different geographical regions. The measurements are called 'variables' and can be of dissimilar types (e.g. GDP and birth rate).

Scatterplots

A data set of this form is often displayed in a scatterplot. A scatterplot shows:

  • the strength of the relationship between the two types of measurement — are large values of one variable associated with large values of the other?
  • individual groups whose values of one variable are unusually high or low in relation to their values of the other variable.

Showing size

When the pairs of measurement are from different geographical regions (e.g. countries), these regions often differ greatly in size. The crosses in a scatterplot can be replaced by coloured circles whose areas are proportional to their sizes (e.g. population or GDP). Circle colour can represent continents or other groupings of the regions.

Nonlinear scales (advanced)

Some variables consist of many small measurements and a few much larger ones. The larger values can dominate a scatterplot, so a logarithmic scale is occasionally used to distinguish the smaller values. However a nonlinear scale should only be used in publications targetted at a more numerate audience.