Grammar Bias Index: Relationships Between Grammar, Age and Vocabulary in CDI Data

Research over the past decades has demonstrated that the development of children’s early grammar skills is closely related to the expansion of their vocabulary. However, recent studies have shown that children’s age plays a separate role in the development of grammar skills, beyond its role as a driving force for vocabulary growth, and that it is possible to distinguish between children’s vocabulary and grammar as two overlapping but different types of knowledge. In this paper, we look more closely at relationships between grammar results, age, and the knowledge of individual words in a Norwegian MacArthur-Bates Communicative Development Inventories (CDI): Words and Sentences dataset. We explore whether words can be identified as more closely associated with higher or lower grammar scores relative to vocabulary size among children aged 1;4 to 3;0. To this end, a Grammar Bias Index (GBI) of all words in the Norwegian CDI vocabulary was defined and calculated, ranging from −1 to 1. The words with the highest GBI tend to be function words, whereas words with the lowest GBI tend to be concrete nouns. Content words have a significantly lower GBI than function words, and nouns have a significantly lower GBI than verbs and adjectives. Frequency had little or no impact on GBI within word class. Controlling for vocabulary size, high-grammar children tend to be older than low-grammar children. The results indicate that this new measure provides insight into the relationships between age, vocabulary and grammar, and we believe that it can be extended to other sets of data to yield further theoretical insights.