in Book - Make it stick.. the science of successful learning, it is argued that interleaving different kinds of practice of a skill (or even a different skill) leads to better learning outcomes than the same kind of practice (referred to as "massed" practice)
A group of eight-year-olds practiced tossing beanbags into buckets in gym class. Half of the kids tossed into a bucket three feet away. The other half mixed it up by tossing into buckets two feet and four feet away. After twelve weeks of this they were all tested on tossing into a three-foot bucket. The kids who did the best by far were those who’d practiced on two- and four-foot buckets but never on three-foot buckets. - R. Kerr & B. Booth, Specific and varied practice of motor skill, Perceptual and Motor Skills 46 (1978), 395–401.
The kids in the study performed better even in the category that they never practiced before, showing how large the effect of interleaving is.
It is hardly surprising that this fact is unintuitive, as pop culture really drills the idea that massed practice is the best way to improve any given skill with its "all-nighters" and "training montages" (see The most effective learning strategies are not intuitive).
Another point raised in the book that explains the unpopularity of interleaved practice: Massed practice gives rapid, short term results while interleaving impedes performance during initial learning. This makes learning feel slow and unrewarding, even though it is vastly more efficient in the long run. The authors point to the following article, where this is further discussed:
N. C. Soderstrom & R. A. Bjork, Learning versus per for mance, in D. S. Dunn (ed.), Oxford Bibliographies online: Psychology (New York: Oxford University Press, 2013) doi 10. 1093/obo/9780199828340-0081.
The authors present a paper that argues that learning through massed practice is encoded in a simpler representation and can be applied less broadly than learning gained through spaced, interleaved practive:
S. S. Kantak, K. J. Sullivan, B. E. Fisher, B. J. Knowlton, & C. J. Winstein, Neural substrates of motor memory consolidation depend on practice structure, Nature Neuroscience 13 (2010), 923– 925
They argue that this flexible representation learned in interleaved practice helps in discriminating between varying problems and contexts when looking for a suitable solution, which is a skill needed both for the more tricky exam questions and real life. You do not have practice in the process of sorting solutions in your head, as the suitable solution was most often the one you just studied moments ago.