Abstract
We report the results of a preregistered classroom experiment ( https://aspredicted.org/x25h-d427.pdf ) investigating the immediate and long-term effects of interleaved practice for learning spelling rules among German third graders (N = 147). The study also investigated whether instructional guidance-comprising prompts and explanations that highlight key features and direct attention to relevant differences between concepts-enhances the effectiveness of interleaving by supporting comparison processes. Children completed two training sessions practicing words governed by specific spelling rules (capitalization, single and multiple consonants, words with "i" and "ie," and words with "ss" and "ß"), with one session conducted in a blocked and the other in an interleaved format. Children made fewer spelling errors on words practiced under the interleaved condition compared to the blocked condition, both immediately after training and at an 8-week follow-up. Instructional guidance influenced performance on new, unpracticed words governed by the same spelling rules. However, its effectiveness was moderated by children's prior knowledge: instructional guidance supported children with low prior knowledge during blocked practice in the immediate posttest and benefited children with high prior knowledge during interleaved practice at follow-up. These findings suggest that interleaved practice is an effective strategy for promoting lasting learning of spelling rules and facilitating transfer, though primarily among children with high prior knowledge. Future research should investigate whether providing children with a broader knowledge base through explicit instruction before the practice phase can help children with low prior knowledge to benefit more fully from interleaving.