Research on the effects of second language strategy instruction (SI) has been extensive yet inconclusive. This meta‐analysis, therefore, aims to provide a reliable, quantitative measure of the effect of SI as well as a description of the relationship between SI and the variables that moderate its effectiveness (i.e., different learning contexts, treatments, and outcome variables). A comprehensive search was conducted to collect the population of SI studies. Effect sizes were calculated for 61 primary studies, contributing a total of 95 unique samples, all of which were coded for potential moderators. The findings indicate a small to medium overall effect of SI (d = 0.49). Variables found to moderate its effectiveness include type and number of strategies, learning context (second vs. foreign language), and length of intervention. Following a contextualized interpretation of the results, the article concludes with a discussion of theoretical, practical, and methodological implications.
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