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

It is important to note several limitations of the present study. First, research on incidental vocabulary learning tends to include careful control of learning conditions. This has methodological value because it increases the likelihood that findings can be attributed to the learning conditions. However, it also means that the results may not reflect how words are typically learned incidentally inside and outside of the classroom. For example, within studies, participants typically do not have access to the support that they would typically have within and outside of the classroom. For example, during meaning-focused learning, students may consult dictionaries, teachers, classmates, and parents to aid comprehension of L2 input. Thus, the results found in studies of incidental vocabulary learning studies may be less than those that occur in less controlled conditions. Second, incidental vocabulary learning studies reveal gains that occur for target items. These target items are typically low frequency words that are encountered more often than the other low frequency words within the L2 input. However, low frequency words that are not target items may also be learned to some degree. In addition, higher frequency words that are unknown or partially known may also be learned. Third, the meta-analysis examined gains in knowledge of form recognition and form-meaning connections of words. The degree to which other aspects of knowledge (i.e., spoken form, collocation, word parts, association) may also be gained through exposure to meaning-focused input was not explored. There are relatively few studies examining aspects of knowledge besides form-meaning connection and this is clearly an area where further research is needed.

The meta-analysis also revealed several other areas where research on incidental vocabulary learning is warranted. First, the results showed that there are few studies that include delayed posttests (k = 9) in comparison with immediate posttests (k = 29). We urge readers to be cautious when interpreting the moderator analyses on the delayed tests as further research is necessary to be confident about the generalizability of these findings. The lack of studies including delayed posttests may be owing in part to practice effects occurring from immediate posttest to delayed posttests (e.g., Webb et al., Reference Webb, Newton and Chang2013). There are two options that might be considered to avoid this issue. First, target items could be counterbalanced with half assessed in immediate posttests and the remaining items assessed in delayed posttests. Another option would be to eliminate immediate posttests and assess learning only on delayed posttests. It would be essential to include a no-treatment control group in such a design to control for outside learning. A second area where further research is needed is incidental learning through listening, reading while listening, and viewing. The number of studies that examined immediate posttest gains made through reading (k = 17) was the same as the combined number of studies of viewing (k = 9), listening (k = 6), and reading while listening (k = 2). In addition, there is also a need for more studies investigating incidental learning with younger participants. There were relatively few studies with participants in primary (k = 5) and secondary school (k = 6) in comparison with university (k = 17).