Why prior knowledge could change what spacing does
Spaced practice means spreading review of material over time instead of cramming it into one session. The idea is that each return to the material, after some forgetting has set in, strengthens memory more than a single long exposure would [1]. For that to work, the learner has to be able to re-engage with the material on each return. If the material is too unfamiliar, a second session may be spent rebuilding basic understanding rather than reinforcing it.
That is the intuition behind the concern about prior knowledge. A review of spacing and retrieval practice describes learning as depending partly on what the learner already brings to the task, including prior knowledge and prior thoughts, which shape how new material is processed [4]. In that framing, spacing is not a fixed dose that works equally for everyone; its payoff depends on what the learner can do with the material when they meet it again.
What classroom and college studies actually show
A study of spaced retrieval practice in college mathematics makes the concern concrete. The authors noted that, before running the comparison, it seemed conceivable that differences in students' prior knowledge and study habits within each group could wash out any effect of spacing between groups [2]. In other words, variability in what students already knew was treated as a real threat to detecting the benefit, not as a side issue. The study is a caution about measurement as much as a finding about prior knowledge itself.
Classroom-focused work on spaced repetition likewise frames prior understanding as part of the picture, presenting spacing as a method whose results depend on the learner's starting point [1]. And a discussion of spaced retrieval in legal education describes spacing as offering benefits both for direct factual recall and for applying information, while noting that some learning tasks require building on prior understanding [3]. That is a claim about what kinds of material spacing can serve, not a measured comparison of stronger and weaker learners.
The direct test has not been reported here
The gap is worth stating plainly. The college math study raised prior knowledge as a possible source of noise but did not report comparing spacing benefits across students with different levels of prior knowledge [2]. The reviews and classroom discussions treat prior understanding as relevant context rather than as a variable whose effect on spacing was isolated [1][3][4]. A large multisite study of retrieval and spaced practice in introductory psychology classrooms examined long-term outcomes but did not address prior understanding at all [5], so it cannot speak to this question either way.
What the evidence supports, then, is a reasonable expectation rather than a demonstrated rule: learners who already understand the material are plausibly better positioned to profit from repeated, spaced returns to it, because each return can deepen rather than restart their understanding [4]. Whether that expectation holds, and how large the difference is, would require studies that measure prior knowledge and spacing benefit in the same learners.
What a learner or teacher should take from this
The practical implication is modest but useful. Spacing is well supported as a way to improve retention, including for vocabulary and for applying information, not just for memorizing facts [3][4]. But it is not a plug-in fix that erases differences in background knowledge. If a learner is meeting genuinely new material, the first pass may need to focus on building understanding, with spaced review doing its work afterward.
For teachers, the college math example is a reminder that a class with wide variation in prior knowledge can make the average benefit of spacing look smaller than it is for any particular student [2]. Tracking who gains and who does not, rather than only whether the class as a whole improved, would be the way to see whether prior understanding is shaping the result.
Sources used in this answer
Spaced practice in classrooms
Classroom work on spaced repetition frames spacing as a method whose results depend on learners' prior understanding.
Spaced retrieval practice increases college students' short-and long-term retention of mathematics knowledge
In a college mathematics study, the authors noted that within-group variability in students' prior knowledge and study habits could conceivably wash out between-group effects of spaced retrieval practice.
Learning Theory and the Law: Spaced Retrieval and the Law School Curriculum
Spaced repetition is described as offering benefits for both direct factual recall and applying information, with some learning tasks requiring building on prior understanding.
The science of effective learning with spacing and retrieval practice
A review of spacing and retrieval practice describes learning as shaped partly by what the learner already brings, including prior knowledge and prior thoughts.
A longitudinal multisite study of the efficacy of retrieval and spaced practice in introductory psychology.
A multisite classroom study examined the long-term impact of retrieval and spaced practice in introductory psychology but did not address learners' prior understanding.