Interleaving is a very effective tool in every teachers’ toolbox, but this new meta-analysis shows some interesting nuances:

Interleaving is clearly effective for inductive learning of complex visual material with high similarity between categories but not within categories. In contrast, results concerning the interleaving effect for learning with expository texts are mixed. Thus, interleaving should be used with caution when using this type of materials. For mathematical tasks, a small overall effect was found, and the effects were overall heterogeneous, indicating the need for further research to clarify the conditions that are crucial for interleaving to be effective with these types of learning materials. In sum, interleaved learning shows great potential, but the type of learning materials and the category structure that characterizes the learning materials need to be taken into consideration for practical implementations.

Abstract of the meta-analysis:

An interleaved presentation of items (as opposed to a blocked presentation) has been proposed to foster inductive learning (interleaving effect). A meta-analysis of the interleaving effect (based on 59 studies with 238 effect sizes nested in 158 samples) was conducted to quantify the magnitude of the interleaving effect, to test its generalizability across different settings and learning materials, and to examine moderators that could augment the theoretical models of interleaved learning. A multilevel meta-analysis revealed a moderate overall interleaving effect (Hedges’ g = 0.42). Interleaved practice was best for studies using paintings (g = 0.67) and other visual materials. Results for studies using mathematical tasks revealed a small interleaving effect (g = 0.34), whereas results for expository texts and tastes were ambiguous with nonsignificant overall effects. An advantage of blocking compared with interleaving was found for studies based on words (g = −0.39). A multiple metaregression analysis revealed stronger interleaving effects for learning material more similar between categories, for learning material less similar within categories, and for more complex learning material. These results are consistent with the theoretical account of interleaved learning, most notably with the sequential theory of attention (attentional bias framework). We conclude that interleaving can effectively foster inductive learning but that the setting and the type of learning material must be considered. The interleaved learning, however, should be used with caution in certain conditions, especially for expository texts and words.