Two types of synchronic data are significant in comparing the productivity of different formations: their relative frequency of occurrence and their relative regularity. The two are of course interrelated. [...] If two alternants have approximately the same frequency of occurrence, but if one of them combines usually with forms that exhibit considerable modification of their underlying forms, while the other combines with unmodified forms, the latter is likely to illustrate the more productive pattern, and - other things being equal - should be chosen as the basic alternant.
- Nida (1948), a pag.426-427