Fortunately, if a condition of vocabulary balance does exist in a given sample of speech, we shall have little difficulty in detecting it because of the very nature and direction of the two Forces involved. On the one hand, the Force of Unification will act in the direction of decreasing the number of different words to 1, while increasing the frequency of that 1 word to 100%. Conversely, the Force of Diversification will act in the opposite direction of increasing the number of different words, while decreasing their average frequency of occurrence towards 1. Therefore number and frequency will be the parameters of vocabulary balance.
Since the number of different words in a sample of speech together with their respective frequencies of occurrences can be determined empirically, it is clear that our next step is to seek relevant empiric information about the number and frequency of occurrences of words in some actual samples of speech.
(George K. Zipf, Human Behavior and the Principle of Least Effort, 1949)