Theoritical aspects

Measuring morphological productivity
There are basically two approaches to measure morphological productivity:
A dictionary-based method (counting neologisms in a certain period of time by using a dictionary; e.g. you can count how many new English words ending in –ian were formed in the 20th century)
A corpus-based method (finding words that occur only once in a large text corpus, so-called hapax legomena; these hapaxes are an indicator of the word-building power of an affix or another word-forming element).
We’ll focus our attention on the first method.
R. S. Ginzburg says that productivity of derivational patterns and affixes should not be identified with the frequency of occurrence in speech, although there may be some interrelation between them. Frequency of occurrence is characterized by the fact that a great number of words containing a given derivational affix are often used in speech, in particular in various texts. Productivity is characterized by the ability of a given affix to make new words.
The dictionary-based method that is probably the most widely used and the
most widely rejected at the same time.
According to this measure, the productivity of an affix can be discerned by counting the number of attested different words with that affix at a given point in time. This has also been called the type-frequency of an affix.
The severe problem with this measure is that there can be many words with a given affix, but nevertheless speakers will not use the suffix to make up new words. An example of such a suffix is -ment, which in earlier centuries led to the coinage of hundreds of then new words. Many of these are still in use, but today’s speakers hardly ever employ -ment to create a new word and the suffix should therefore be considered as rather unproductive (cf. Bauer 2001:196). Thus the sheer number of types with a given affix does not tell us whether this figure reflects the productivity of that affix in the past or its present potential to create new words.
Counting derivatives can nevertheless be a fruitful way of determining the
productivity of an affix, namely if one does not count all derivatives with a certain affix in use at a given point in time, but only those derivatives that were newly coined in a given period, the so-called neologisms. In doing this, one can show that for instance an affix may have given rise to many neologisms in the 18th century but not in the 20^th century. The methodological problem with this measure is of course to reliably determine the number of neologisms in a given period. For students of English this problem is less severe because they are in the advantageous position that there is a
dictionary like the Oxford English Dictionary (OED). This dictionary has about 500,000 entries and aims at giving thorough and complete information on all words of the language and thus the development of the English vocabulary from its earliest attestations onwards.

Affix	OED Neologisms
-able	185
-full (measure)	22
-full (property)	14
-ion	625
-ish	101
-ist	552
-ity	487
-ize	273
-less	103
-ness	279
-th	2
-ter	1
-wise	12
-er	312

To summarize our discussion of how productivity can be measured, it should have become clear that the different measures have the great advantage that they make
certain intuitive aspects of morphological productivity explicit and calculable.
Furthermore, we have learned that productivity is largely a function of the frequency of words and that the reason for the connection between frequency and productivity lies in the nature of the storage and processing of (complex) words in the lexicon.