The component must then be represented by a visual variable in which a doubling the variable's physical magnitude produces a doubling of perceived magnitude. Brightness could not be used, for example, because doubling intensity produces only about a 1.4 factor increase in perceived brightness. Bertin suggests that such failure to properly match the component and visual variable level of organization is the major single source of error in visualization design . Original Bertin Associative Organization Bertin subdivides the nominal scale level into two subcategories, associative and selective. Associative perception is the lowest organization level, allowing grouping of all elements of a variable in spite of different values.
Shape is associative because the observer automatically groups different shapes together.
If you look away and then glance quickly back at the graph, data points group by brightness, and shape differences are ignored. On the other hand, brightness is not associative because observers cannot easily ignore different values. In Figure 4, it is much harder to distinguish different shapes and to ignore brightness differences. As result, viewers will find it much more difficult to visualize the relationship among height, weight and gender. It permits the viewer to select one category of a component, perceive locations of objects in that category and ignore others.