Therefore, in language dynamics, we shall have to be satisfied with determining in what direction the variation of certain factors is likely to cause that of the others. These variables are (1) the number of the units among which the speaker chooses at a given point of the utterance, (2) the probability of units, which we shall in practice identify with their frequency, either in a definite position or generally, 3) the cost of each unit, and (4) its information.
- Martinet (1962), Pag. 142