[...] productive patterns, formations which cannot be said to have a rigidly limited distribution. Compare the allomorphs of the plural suffix, /-əz ~ -z ~ -s/, vs. /-ən/: the former set is productive, the latter is not.
- Nida (1948), a pag.431

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. Allomorphs that occur in a larger number of combination and in a larger number of actual utterances are likely to illustrate the productive patterns, but regularity of formation is important also. 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