Continuing his series on the potential of retrieval practice, spaced learning, successive relearning, and metacognitive approaches in the classroom, this time Kristian Still focuses on the ‘spaced’ element, looking at the underpinning research and drawing out important lessons for teachers


In this series, I am attempting to elaborate and share what the recipe of test-enhanced learning (more commonly known as retrieval practice), spaced learning, interleaving, feedback, metacognition, and motivation might look like in and out of the classroom.

I am reviewing the research and cognitive science behind these concepts and the modulators underpinning the effective retention of knowledge.

In writing this series, nine clear but interlinked elements emerged, and so I am considering these elements across nine distinct but related articles:

I would urge readers to also listen to a recent episode of the SecEd Podcast (SecEd, 2022) looking at retrieval practice, spaced learning, and interleaving and featuring a practical discussion between myself, teacher Helen Webb, and Dr Tom Perry, who led the Education Endowment Foundation’s Cognitive Science Approaches in the Classroom review (Perry et al, 2021).

This series, in reviewing the evidence-base, seeks to help you reflect on what will work for you, your classroom, and your pupils. This is article three and it focuses on spaced retrieval practice or spaced learning.


Spaced retrieval practice

“Much of what is learned during a first exposure is forgotten during the interval between exposures and must be relearned after to become a part of semi-permanent knowledge. One can, therefore, conceive of the total acquisition process as a cycle of acquisition, loss, and re-acquisition of information.”
Bahrick, 1979

“Spaced practice applies the principle that material is more easily learnt when broken apart by intervals of time.”
Perry et al, 2021


Spacing involves taking the given amount of time that is devoted to learning and arranging that into multiple sessions spread over time. In this way, the learning sessions are said to be “spaced” or “distributed”. This as opposed to blocked or massed practice, where the learner does all their studying in one session.

Spacing was the second teaching technique to receive a high "utility" rating in the landmark Dunlosky et al study (2013) – if you remember, "practice testing" was the other (see article two).

And as Dr Tom Perry highlighted in the SecEd Podcast episode (2022) accompanying this series: “Repeatedly in research, we find that rather than doing a whole hour at once, you are best off doing 15 minutes four times.”

Researchers believe that spacing works because the time delay between study sessions forces the learner to work harder to retrieve the information and that this helps to consolidate information in long-term memory.

However, spacing makes things harder for the learner, which in itself can present challenges for the teacher to overcome (as we will discuss later in this series).

But as Perry et al (2021) state: “While spaced practice is thought to make learning more challenging for pupils as it prohibits information being held in the working memory, it may be able to increase the likelihood of knowledge being embedded in pupils’ long-term memory.

“In having pupils revisit key concepts, ideas, or skills over longer periods of time in which content is almost forgotten, teachers may be able to improve learning retention.”

Studies show that distributing a fixed amount of practice over multiple sessions can boost scores on a delayed test (Dunlosky et al, 2013; Kang, 2016; Karpicke et al, 2016; Pyc & Rawson, 2012; Yan et al, 2016).

Cepeda et al (2006) looked at 254 studies with 14,000 participants and reported that students scored about 10% higher on tests after spaced study than after massed study. More recent classroom-based randomised studies have supported these findings (Nazari & Ebersbach, 2019; Emeny et al, 2021; Lyle et al, 2020).

Last year, Professor John Dunlosky told the Tes Podcast (2021): “Spacing one’s activities, and repeating those activities, is the basis of all mastery. If students are going to master anything, you are going to have to repeat that content over and over again – there is no getting around it.”

And Bjork and Bjork (2020) define it as “one of the most reliable and extensively studied phenomena in the field of learning”.

You may also be interested to know that Emeny et al (2021) report that spaced practice (generally) not only produces higher test scores than massed practice, but students’ predictions of their own test scores were relatively accurate after spaced practice yet grossly overconfident after massed practice. This suggests that spaced practice improves metacognitive monitoring or, alternatively, addresses our own illusions of competence – a theme I come back to in article eight.

Yet there is a very obvious glitch. Spaced practice is not the default study habit for most students (particularly those who are performing poorly). The subjective sense of fluency engendered by massed practice misleads students into feeling/believing large gains in learning (Finn & Tauber, 2015).

And dare I say it, colleagues, nor is spaced practice the default model for teaching…

And yet introducing spacing techniques doesn’t need to be overwhelming, as teacher Helen Webb told the SecEd Podcast (2022): “What is the work that needs revisiting? I don't think we need to talk about re-sequencing your entire curriculum model – just think more purposefully about what it is you are doing in the classroom and different ways to revisit previous learning.”

Speaking to me after the podcast, Helen offered some further encouragement: “Teachers naturally encourage students to revisit previous material during 'Do Now' starter quizzes, recapping previous material as part of an explanation of 'new' material, via homework, in lessons going over previous classwork and homework, during DIRT activities or scheduled regular review or SLOP (shed loads of practice) lessons – e.g. Flash Back Fridays!”


Embracing spacing

“More studies have examined the impact of spacing content across days and lessons than within individual lessons. Regarding the former, the evidence suggests a small positive impact across pupil ages and subject areas.”
Perry et al, 2021


So, what kind of learning gains are we talking about? Well, practising retrieval once doubles long-term retention relative to reading the text once (34% vs 15%) and engaging in repeated retrieval increases retention to 80% (Karpicke & Roediger, 2010).

Lyle et al (2020) report similar results: “Students for whom some retrieval practice was spaced, compared to those for whom all practice was massed, performed better on the final exam in the precalculus class and on exams in the calculus class.”

There is a warning, of course, as I touched upon in article two when I discussed the fine line between difficulty and motivation: as the lag between retrievals and an initial test increases, the likelihood of forgetting also increases. Thus, spaced retrieval practice is a prime example of how retrieval success and retrieval effort trade-off and why balancing these two components is crucial for promoting learning (just as crucial as providing corrective feedback, as we have already said in this series, and will say again in article five on feedback).

Neuroscience expert Sarah Cottingham explained: “The longer the spacing interval the more forgetting sets in and therefore the more challenging it is to retrieve the information. Effort is important but so is successful retrieval. We have to use spacing in a way which delicately balances effort with success for pupils.”


Quick solutions: Teachers can take different approaches depending on what their aims are – perhaps you’d like lower failure rates (increased success) as an entry into learning or perhaps you want to increase the challenge of retrieval tasks in order to strengthen storage in long-term memory…


Effort is vital and we must warn students of this fact: "Human memory is fragile. The initial acquisition of knowledge is slow and effortful. And once mastery is achieved, the knowledge must be exercised periodically to mitigate forgetting.” (Lindsey et al, 2014)

Very simply, the more occasions knowledge is tested, the larger the learning gains. According to Yang et al (2021), learning gains increase the more we test knowledge, but of note for teachers is that the extent of the returns diminishes: one retrieval quiz (an effect size of 0.44), two quizzes (0.60), three retrievals or more (0.64), and unlimited retrievals (0.76). Perhaps three is the magic number. Or at least a good bet. We will come back to this in article six (successive relearning).


Effects of the type of spacing schedule

Two broad schedules have frequently been compared: expanding spacing versus uniform spacing. For example, we could test or return to the target information after one lesson, then after another five lessons and then after another nine lessons (1-5-9), or we could return to the target information every five lessons (5-5-5). In the expanding schedule, intervals increase after every re-exposure, whereas in the uniform schedule intervals are kept constant.

Of course, a commonly used classroom strategy is to start the lesson with a short retrieval quiz that not only strengthens pupils’ prior learning but also activates the relevant knowledge pertinent to that lesson and consequently reduces the cognitive load for the proceeding taught episode.

Practically, as Perry et al (2021) outlined, spacing is most often applied “across days and lessons” than within a lesson,however it can be across weeks, terms or across topics.

The idea behind an expanding schedule is that early recalls would afford high success while the retrievals grow increasingly more difficult (because of the increased spacing). The notion is intuitive and for years it was recommended as the optimal way.

However, a wealth of research since Cepeda et al’s meta-analysis (2006) has failed to demonstrate a consistent advantage of expanding over uniform schedules.

Most recently, Latimier et al (2021) challenged the belief that spacing schedules should be progressively increased. Interestingly, what they also pointed out was that “the more learners are tested, the more beneficial the expanding schedule is, compared with the uniform one”.

So, just how do we answer that most crucial of questions: what is the optimal retrieval practice interval? Well, rather disappointingly, the answer is: “It depends…”

  • It depends on “the memory strength of that item after initial encoding” (Cepeda et al, 2009). If stronger, extend the spacing and conversely where it is weaker or incorrect reduce the spacing.
  • It depends “on the level of initial training moderating the effects of distributing retrieval practice” (Toppino et al, 2018; Yang et al, 2021). So again, what was the initial learning? One test, a whole mind-mapping exercise, repeated exposures? How well learnt, secured, and consolidated is that training? If it has been learned in-depth, we can have longer spacing.
  • It depends on “the time elapsed since the last review” (Tabibian et al, 2019). This relates to our expectations of success, which will be moderated by the time between learning and retrieval. And quite frankly, this is dictated by the timetable. If you see your students once a week, they will find retrieval exercises more difficult than if you see them every day (although there’s always homework...).
  • And lastly, if critically, it depends on “how long you wish to remember something” (Cepeda et al, 2008). If the information is required for an extended period of time, reinforce both the storage and retrieval strength. But as a general rule, if the goal is long-term retention, review information several weeks or months after it was initially learned. If the test is in a month, review the information around once a week. If the test is in a week, review the information daily.

Keep in mind that shorter delays between studying sometimes fool the learner into thinking they know the information well (for long-term retention) because the information is more easily retrievable, but alas information is quickly forgotten.

However, there is some good news: “The cost of overshooting the right spacing is consistently found to be much smaller than the cost of having very short spacing.” (Pashler et al, 2007). In other words, err on the side of a longer interval.


Not all retrieval is good retrieval (interference)

An ironic feature of human memory is that while the act of remembering a target piece of information may strengthen the retrieved information itself, it can cause forgetting or weakening of other related information stored in long-term memory. This phenomenon is known as “retrieval-induced forgetting”.

One of the reasons for this will be “interference”, when competition from other items produces forgetting (Anderson & Neely, 1996) – the related information is still there in long-term memory, but the interference is making it temporarily difficult to retrieve.

The other items may have been learned before or after the target item (proactive or retroactive interference). Proactive interference means you may struggle to remember your new mobile phone number while retroactive interference means you may, when learning a new piece of music, suddenly find it more difficult to play an older, previously learnt piece of music.

Teachers will benefit from being aware of this phenomena when planning lessons: we should avoid pupils encountering similar information too soon after they encounter the target material – “wait a little while for forgetting” (Storm et al, 2010).


Sleep tight

We all know how vital sleep is. One of the key aspects of spacing is that it allows for sleep. While we can learn without sleeping between learning sessions, for long-term retention sleep is unquestionably advantageous. Not only that, it requires no cost and no additional effort.

Sleeping provides optimal conditions for the processes that integrate newly encoded memories into long-term storage (sometimes referred to as consolidation) and sleep also “enhances new learning” (Diekelmann & Born, 2010).

Memories are re-activated during sleep and where these new memories would be prone to decay, instead they are transformed into more stable memories that are preserved long-term (van Dongen et al, 2012).

Sleep then not only helps to strengthen memories, but also helps us to actively forget irrelevant information, thus optimising memory for what is relevant.

Such transformations allow learners to re-encode information faster and to save time during the relearning sessions. The practical consequence reported by Mazza et al (2016) was that sleep between learning sessions reduced the amount of practice needed to relearn 16 paired words by half, and also ensured better long-term retention. Their recommendation: “Sleeping after learning is definitely a good strategy but sleeping between two learning sessions is a better strategy.”

Meanwhile, Kroneisen and Kuepper-Tetzel (2021) tell us that if students study in the evening, they should test themselves immediately after learning. If students study during the day, the practice test should be delayed in order to reinforce memory and reduce forgetting.


Implications for the classroom

Despite more than a century of research demonstrating the spacing effect, it does not appear to have widespread application in the classroom. Conventional instructional practice, organised in a modular way, typically makes massed practice more convenient.

Teachers deciding to incorporate spaced practice or spaced retrieval practice into their teaching need to be encouraged to break free of these conceptual constraints. And remember, when pupils are re-exposed to information that they have learned but temporarily cannot recall, they acquire this information much faster than information that is being learned for the first time.

As Kang (2016) concluded: “Spaced review or practice enhances diverse forms of learning, including memory, problem-solving, and generalisation to new situations. Spaced practice is a feasible and cost-effective way to improve the effectiveness and efficiency of learning, and has tremendous potential to improve educational outcomes.”

In a nutshell, spacing reduces retrieval strength but boosts storage strength. Spacing reduces the negative effects of fluency (the success experienced during blocked practice) and increases the effort required to retrieve information, making learning harder but more durable.

Furthermore, as well as directly consolidating that knowledge, learners (and their teachers) are able to diagnose what they know and identify gaps in their knowledge.

Maybe, the last word should be from a teacher who has crossed the research divide. William Emeny teaches maths at Wyvern College in Hampshire and co-authored a paper entitled Spaced mathematics practice improves test scores and reduces overconfidence (Emeny et al, 2021).

He writes: “The key point is that teachers should shift their mindset so that the practice of a skill or concept is seen not as material that should be squeezed into one or two consecutive class meetings, but rather as material that can be distributed across many lessons.”


Takeaways

  • Spaced over massed practice (be it retrieval or not).
  • Combine spacing with retrieval and consider the benefits of interleaved retrieval as well (see article four).
  • Optimal spacing needs to consider: The memory strength of initial encoding and how long you wish to remember something (when is the exam?!).
  • Overshooting the right spacing intervals is less harmful than spacing that is too short.
  • Spacing across days and lessons is more effective than within lessons – although limited research into the latter is promising (Perry et al, 2021).
  • Consider encoding and relearning as separate and distinct retrieval opportunities.
  • Three practice tips from Carpenter et al (2012): Incorporate into each lesson a brief review of concepts that were learned several weeks earlier; use homework to “re-expose” important information; set exams and quizzes that are cumulative.
  • “Spacing within the confines of the existing curriculum and timetable may … be a more feasible application for classroom teachers to implement themselves.” (Perry et al, 2021)
  • Sleep is a spacing opportunity.
  • Homework is a spacing opportunity.
  • Be aware that retrieval can, at times, interfere and/or inhibit remembering.
  • And if you only have time to read one paper on this topic: A meta-analytic review of the benefit of spacing out retrieval practice episodes on retention (Latimier et al, 2021): https://bit.ly/3u8cb3m


  • Kristian Still is deputy head academic at Boundary Oak School in Fareham. A school leader by day, together with his co-creator Alex Warren, a full-time senior software developer, he is also working with Leeds University and Dr Richard Allen on RememberMore, a project offering resources to teachers and pupils to support personalised spaced retrieval practice. Read his previous articles for SecEd via https://bit.ly/seced-kristianstill


References: For all research references relating to this article, go to https://bit.ly/3OjCYSb

Acknowledgement: This article would not have been possible without the author’s conversations with Dr Dan Rosen, head of secondary at St George's School in Düsseldorf, Germany (@musingsofadr). The opportunity to discuss the test-enhanced learning research from a school leader's point of view added a welcome layer of depth and professional challenge.

ResearchED: Kristian will be speaking at the first ever ResearchED Berkshire taking place at Desborough College in Maidenhead on May 7. Visit https://researched.org.uk/event/researched-berkshire/

RememberMore: RememberMore delivers a free, personalised, and adaptive, spaced retrieval practice with feedback. For details, visit www.remembermore.app or try the app and resources via https://classroom.remembermore.app/