The desired approximation would have to disregard some of the differences in distribution among morphemes, i.e. if it groups together two particular morphemes into one morpheme class, that would not mean that every one of the total environments of the first of these is completely identical with an environment of the other; this means that the total distribution of the first is not necessarily identical with the total distribution of the second. […].
- Harris (1951), a pag.244

The most direct approximation to classes of identically distributed morphemes would seem to be the grouping together of morphemes which are identical in respect to some stated large fraction of all their environments. To perform this approximation, we take each morpheme and state all of its environments within the corpus, where the environment is taken to be the whole utterance in which it occurs. We then select one morpheme, and match its range of environments with that of each other morpheme. We do not expect to find many cases of identical ranges, but decide instead upon certain conditions; if a morpheme satisfies these conditions, it will be assigned to the class of our originally selected morpheme. The conditions may vary with the language system and with our purposes. They may vary with the language system and with our purposes. They may be as crude as requiring that 80 per cent of the environments of the one morpheme should be ones in which the other also occurs. Or the conditions may require that particular types of difference apply among the environments in which the two morphemes do not substitute: e.g. that the morphemes in which the two environments differ be themselves members of one class by the present method. Thus if hear and tear occur, without being substitutable for each other, in 'I’ll-the bell' and 'I’ll-the paper', respectively, our conditions might require that 'bell' and 'paper' be assignable to one class in terms of this same method of approximation.
- Harris (1951), a pag.245

The method of approximation most commonly used by linguists today is the consideration of environments shorter than the full utterance. A limited stretch of each utterance is selected, and morphemes are grouped together into a class if they can replace each other in that limited environment. Thus we might select the position '–ly' to yield the class of 'large', 'clean', 'true', etc. Similarly, the environment 'the-' or 'the large-' might be used to yield the class of 'man', 'auto', 'life', etc.
- Harris (1951), a pag.255