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.
- Nida (1948), a pag.426-427

The very frequency of occurrence of a form also contributes a feature of linguistic meaning, for it is important on the one hand in establishing patterns of analogy, and on the other in preserving the form against levelling influence. Irregular forms of frequent occurring words (e.g. the forms of the verb 'to be' in English) resist levelling analogies which extend to other verbs of less frequent occurrence.
- Nida (1948), a pag.430