Associative lexical cohesion as a factor in text complexity

In this paper we present an application of associative lexical cohesion to the analysis of text complexity as determined by expert-assigned US school grade levels. Lexical cohesion in a text is represented as a distribution of pairwise positive normalized mutual information values. Our quantitative measure of lexical cohesion is Lexical Tightness (LT), computed as average of such values per text. It represents the degree to which a text tends to use words that are highly inter-associated in the language. LT is inversely correlated with grade levels and adds significantly to the amount of explained variance when estimating grade level with a readability formula. In general, simpler texts are more lexically cohesive and complex texts are less cohesive. We further demonstrate that lexical tightness is a very robust measure. We compute lexical tightness for a whole text and also across segmental units of a text. While texts are more cohesive at the sentence level than at the paragraph or whole-text levels, the same systematic variation of lexical tightness with grade level is observed for all levels of segmentation. Measuring text cohesion at various levels uncovers a specific genre effect: informational texts are significantly more cohesive than literary texts, across all grade levels.