Zipf's law revisited: Spoken dialog, linguistic units, parameters, and the principle of least effort.

The ubiquitous inverse relationship between word frequency and word rank is commonly known as Zipf's law. The theoretical underpinning of this law states that the inverse relationship yields decreased effort in both the speaker and hearer, the so-called principle of least effort. Most research has focused on showing an inverse relationship only for written monolog, only for frequencies and ranks of one linguistic unit, generally word unigrams, with strong correlations of the power law to the observed frequency distributions, with limited to no attention to psychological mechanisms such as the principle of least effort. The current paper extends the existing findings, by not focusing on written monolog but on a more fundamental form of communication, spoken dialog, by not only investigating word unigrams but also units quantified on syntactic, pragmatic, utterance, and nonverbal communicative levels by showing that the adequacy of Zipf's formula seems ubiquitous, but the exponent of the power law curve is not, and by placing these findings in the context of Zipf's principle of least effort through redefining effort in terms of cognitive resources available for communication. Our findings show that Zipf's law also applies to a more natural form of communication-that of spoken dialog, that it applies to a range of linguistic units beyond word unigrams, that the general good fit of Zipf's law needs to be revisited in light of the parameters of the formula, and that the principle of least effort is a useful theoretical framework for the findings of Zipf's law.