Do dictionary definitions support seventh graders' acquisition of word meaning while reading?

Looking up unknown words in a dictionary while reading is a common recommendation at school. However, little is known about the effects of dictionary use on the acquisition of word meaning while reading in the language of instruction. This study investigated whether students acquire the meaning of unknown words by using a dictionary while reading. The aim was also to explore whether students´ knowledge of the target words differ regarding the dictionary used (print or electronic dictionary). We conducted an experiment in which seventh-grade students in Germany (N = 153) were asked to read a text in the language of instruction (German) and look up twelve target words in a dictionary. Subjects were assigned to three treatment conditions (Kindle/integrated electronic dictionary, Duden/print dictionary, control/no dictionary). Knowledge of the target words was tested by means of a vocabulary test twice (one after reading the text and one 14 days later). Best fitting models were calculated using generalized linear mixed models (GLMM). For the first vocabulary test, results showed a significant effect of treatment condition. However, contrasts revealed a significant difference only between the Kindle group and the Duden group and between the Kindle group and the control group. No differences between groups could be found for the second vocabulary test. The results support the use of dictionary definitions for the acquisition of word meanings while reading on electronic reading devices.