In one sense the restrictions on a word-formation rule define the domain where a rule can be productive. From a slightly different point of view, such restrictions [...] may [...] have the effect of concentrating the use of a word-formation rule to a relatively small number of types. [...] Whatever the precise interaction of these factors may be, they all play a part in determining the number of types "V", which we suggest is a measure of the extent of use of a WFR, and they all enter into the assessment of morphological productivity in some more general sense. We will refer to this more general sense of productivity as global productivity. The global productivity "P*" of a WFR can be summarized in terms of its coordinates in the "P-V" plane, with the degree of productivity on the horizontal axis and the extent of use "V" on the vertical axis [...].
- Baayen & Lieber (2004), a pag.77-78