That’s why we should define prefixes not by its derivative or by its relative role but by its positions inside word form, prefix is any morpheme that meets the following requirements:
1) it can be placed only left from nuclear position;
2) it never can be placed upon nuclear position;
3) between this morpheme and nuclear can’t be placed any meaningful morpheme with its clitics (i.e.: between nuclear root and prefix can’t be placed a meaningful morpheme with its auxiliary morphemes).
I am specially to note that there are no so called semi-prefixes. If a morpheme can be placed in nuclear position it is meaningful morpheme and any combinations with it should be considered as compounds.
Thus can be resumed the following:
1) Each language has its own ability to produce prefixation and this ability doesn’t change seriously during all stages of its history.
2) Prefixation ability is manifested by any means: by means of original morphemes existing in a certain language or by borrowed morphemes. That’s why the method doesn’t suppose distinction between original and loaned affixes.
3) Genetically related languages are supposed to have rather close values of Prefixation Ability Index.
2.1.3. PAI calculation algorithm
How Prefixation Ability Index (here and further in this text abbreviation PAI is used) can be measured?
Value of PAI is portion of prefixes among affixes of a language.
Hence, in order to estimate portion/percentage of prefixes of a certain language we should do the following:
1) Count total number of prefixes;
2) Count total number of affixes;
3) Calculate the ratio of total number of prefixes to the total number of affixes.
Why is it important to count total number of prefixes and then calculate the ratio to the total number of affixes but not to estimate PAI by frequency of prefix forms in a random text?
A certain language can have quite high value of PAI but in a particular text word forms with prefixes can be of low frequency. Our task is to estimate portion of prefixes in grammar but not portion of prefix forms in a random text. Prefixes/World index estimated by Greenberg was exactly that estimation of prefix forms frequency in a text (Greenberg 1960).
Of course, that index also can give some general notion of prefixation ability of a language, though it is extremely rough and inaccurate since in a randomly chosen text can be very little amount of words with prefixes: the longer text is the more precision is the conclusion but anyway error of such estimation still remains very high; while when we count all exiting affixes of a certain language potential error is extremely low and even if we occasionally forget some affixes it doesn’t influence seriously on our results.