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"Since the spacing effect was first discovered by Ebbinghaus in 1885, research has consistently shown that learning performance improves if multiple study sessions are separated in time (or “spaced”) rather than massed together. The effect occurs for adults, pre-schoolers and infants, and primary and secondary school children. A brain imaging study suggests the spacing effect in verbal learning is due to enhanced maintenance rehearsal (i.e. additional thinking about the material) in spaced, relative to massed presentations."
https://v1.educationendowmentfoundation.org.uk/uploads/pdf/NSED_LitReview_Final(page 30) file:///Users/Leg/Documents/EEF_Lit_Review_NeuroscienceAndEducation.pdf
More readable references
Review of "Make it Stick" The Science of Successful Learning
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