How effective is second language incidental vocabulary learning? A meta-analysis

The present study revealed that exposure to L2 meaning-focused input contributed to large learning effects for knowledge of form-meaning connections of unknown words. The mean proportions of target words learned ranged from 9–18% on immediate posttests, and 6–17% on delayed posttests. Incidental L2 vocabulary learning gains were similar across reading (17%, 15%), listening (15%, 13%), and reading while listening (13%, 17%) conditions on immediate and delayed posttests. In contrast, the proportion of words learned in viewing conditions on immediate posttests was much smaller (7%, 5%). Moderator analyses revealed that each of learner (proficiency), material and activity (text type, text audience, mode of input), and methodological (pre-knowledge control) variables affected the size of gains on immediate posttests. However, only material and activity (spacing, mode of input) variables moderated gains on delayed posttests. Difference in findings for moderator variables between retention intervals was likely owing in part to fewer studies including delayed posttests.