Is “grammatical diversity” a useful concept?

Introduction

In a recent paper focusing on distributions of simple NPs (Aarts and Wallis, 2014), we found an interesting correlation across text genres in a corpus between two independent variables. For the purposes of this study, a “simple NP” was an NP consisting of a single-word head. What we found was a strong correlation between

  1. the probability that an NP consists of a single-word head, p(single head), and
  2. the probability that single-word heads were a personal pronoun, p(personal pronoun | single head).

Note that these two variables are independent because they do not compete, unlike, say, the probability that a single-word NP consists of a noun, vs. the probability that it is a pronoun. The scattergraph below illustrates the distribution and correlation clearly.

Scattergraph of text genres in ICE-GB; distributed (horizontally) by the proportion of all noun phrases consisting of a single word and (vertically) by the proportion of those NPs that are personal pronouns; spoken and written, with selected outliers identified.

Scattergraph of text genres in ICE-GB; distributed (horizontally) by the proportion of all noun phrases consisting of a single word and (vertically) by the proportion of those single-word NPs that are personal pronouns; spoken and written, with selected outliers identified.

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