Purpose: This study investigates one possible reason for individual differences in vocabulary learning from written context. A Latent Semantic Analysis (LSA) model is used to motivate the prediction of a causal relationship between semantic knowledge for words in a text and the quality of their hypotheses about the semantics of novel words, an effect mediated by reading comprehension. The purpose of this study was to test this prediction behaviorally, using a within subject repeated measures design to control for other variables affecting semantic word learning.
Methods: Participants in 6th grades (n=23) were given training to increase semantic knowledge of words from one of two texts, counterbalanced across participants. After training, participants read untreated and treated texts, which contained six nonword forms. Measures were taken of reading comprehension (RC) and the quality of the readers' hypotheses about the semantics of the novel words (HSNW). Text difficulty and semantic informativeness of the texts about nonwords were controlled.
Results: All participants had increases in semantic knowledge of taught words after intervention. For the group as a whole, RC scores were significantly higher in the treated than untreated condition, but HSNW scores were not significantly higher in the treated than untreated condition. Reading comprehension ability was a significant moderator of the effect of treatment on HSNW. A subgroup of participants with lower scores on a standardized reading comprehension measure (n=6) had significantly higher HSNW and RC scores in the treated than untreated condition. Participants with higher standardized reading comprehension scores (n=17) showed no effect of treatment on either RC or HSNW. Difference scores for RC and difference scores for HSNW were strongly related, indicating that within subjects, there is a relationship between RC and HSNW.
Conclusions: The results indicate that for a subgroup of readers with weaker reading comprehension, intervention to enhance lexical semantic richness had a substantial and significant effect on both their reading comprehension and on the quality of hypotheses that they generated about the meanings of novel words. Neither effect was found for a subgroup of readers with stronger reading comprehension. Clinical and educational implications are discussed.