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

Overall effect of incidental vocabulary learning

In order to examine the overall effectiveness of incidental L2 vocabulary learning through different modes of input, the mean effect sizes along with 95% confidence intervals (CIs) were computed for the first (k = 28) and follow-up (k = 9) posttests. The mean effect sizes for the first and follow-up posttests were large (Plonsky & Oswald, Reference Plonsky and Oswald2014), g = 1.14, 95% CI [0.86, 1.41], p <.001 and g = 0.93, 95% CI [0.44, 1.42], p <.001, respectively. The homogeneity test was statistically significant for both the first and follow-up posttests, Q = 269.08, p <.001 and Q = 55.85, p <.001, indicating the possibility that moderator variables exist accounting for the difference in the magnitude of incidental learning across studies.

Rate of learning

Table 2 shows that for knowledge of form recognition, approximately 18% of the target words were learned on the first posttest, and average retention of form recognition declined very sharply to 6%. The pick-up rates for meaning recognition were 15% on the first posttest and 17% on the second posttest. The rate of learning for meaning recall was the lowest (9%) of the three test formats. Yet, meaning recall appeared to be durable as the rate of learning on the follow-up posttests was slightly higher (12%) than the initial learning rate.

Table 3 presents the learning rates for the four modes of input. Learning rates were highest for reading (17%), followed by listening (15%), reading while listening (13%), and viewing (7%). For the second posttest, it is interesting to note that reading while listening (17%) appeared to help learners retain or enhance word knowledge more than reading (15%) or listening (13%). Viewing appears to be least effective among all modes of input (5%).

Moderator analysis (first posttest)

The results of moderator analyses explore the extent to which the selected moderator variables could account for learning variability across studies. Summaries of results for eight categorical variables for the first and follow-up posttests are presented in Tables 4 and, respectively.

Learner variables (proficiency, institutional level)

For L2 proficiency, the effect size was larger for Beyond basic learners (g = 1.40) than for Basic learners (g = 0.70) and the difference was approaching significance (p =.051). This indicates that more proficient learners are more likely to learn L2 words incidentally than less proficient learners. Regarding institutional level, although not statistically significant, a sizable variation in the effectiveness of incidental vocabulary learning was found across the three groups, indicating that older individuals tended to learn more words than younger ones: university (g = 1.36), secondary (g = 0.87), and primary school (g = 0.72) students.

Material and activity (text type, audience, spacing, mode)

Both text-related variables (text type, text audience) turned out to be significant moderators. Learners exposed to narrative texts (g = 1.43) picked up more L2 words compared with those exposed to expository texts (g = 0.61). Learners exposed to texts designed for L2 learners (g = 1.56) acquired more words than those exposed to texts designed for L1 users (g = 0.71). In addition, the effect size for learning in spaced conditions (g = 1.51) was larger than in massed conditions (g = 0.97), and the difference approached statistical significance (p =.080). With respect to mode of input, significant variation was found across the four modes with a large effect for reading (g = 1.45), medium to large effects for listening (g = 0.97) and reading while listening (g = 0.78), and a slightly lower effect for viewing (g = 0.60).

Methodology (pre-knowledge control, test format)

Significant variation was found across the three approaches to pre-knowledge control of target words revealing a noticeable pattern with studies using pseudowords (g = 1.90) tending to find larger effects compared with studies either pilot testing target words (g = 0.80), administering pretests (g = 1.01), or no pretests (g = 0.27). There was no noticeable difference between test formats: form recognition (g = 1.04), meaning recognition (g = 1.10), and meaning recall (g = 1.08).

Moderator analysis (follow-up posttest)

Overall, the patterns of results for the follow-up posttests were similar to the first posttest results. However, fewer statistically significant differences and effect size values were found owing to smaller sample sizes. The results of three variables (spacing, mode of input, pre-knowledge control) are notable for their contrast to the first posttest results. First, the positive effect of spaced learning (g = 1.71) relative to massed learning (g = 0.58) became even more accentuated in the second posttest (first posttest: Q = 3.06, p =.080 vs. second posttest: Q = 6.22, p =.013). Regarding mode of input, the between-study variation in effect size became non-significant with the results showing a sharp decrease in effect size for reading from the first posttest (g = 1.45) to the second posttest (g = 0.96). In addition, a large effect size for reading while listening (g = 1.07), a medium to large effect size for listening (g = 0.76), and a small effect for viewing (g = 0.43) were found. However, the results for listening, reading while listening, and viewing should be interpreted cautiously owing to the small sample of studies with delayed posttests for these modes of input (k = 2, k = 3, k = 2 respectively). Lastly, it is notable that the effect size for studies in which learners completed pretests appeared to be less subject to change between the first (g = 1.01) and second (g = 1.04) posttests, pointing to the possibility that practice effects occurred in studies with pretest/posttest designs.