Lexical richness viewed through lexical diversity, density, and sophistication

Abstract Recognizing vocabulary as the essential language foundation and determining its richness continues to be a challenge in quantitative linguistics. This issue has also been widely applied in various practical contexts, including language acquisition, language change, psychology, and cognitive science. However, vocabulary is complex, and lexical richness has been typically measured using three distinct indicators: lexical diversity, density, and sophistication. Among them, lexical diversity has been widely studied, yet reliable metrics independent of text length remain lacking. The characteristics of lexical density and sophistication have not been dealt with in depth, and studies focusing on the Japanese language are limited. This study aimed to delineate the distinctive features of lexical diversity, density, and sophistication, and to explore their interrelationships within Japanese. A corpus of formal policy speeches delivered by Japanese prime ministers was utilized. Numerical experiments were conducted in two stages: first, lexical diversity, density, and sophistication were analyzed separately; subsequently, the impact of text length on these indicators was examined. The findings indicated that text length did not affect lexical density and sophistication, and no consistent relationship was observed among the three indicators.