Conceptualising spaced learning in health professions education: A scoping review

Abstract Objectives To investigate the definitions and applications of ‘spaced learning’ and to propose future directions for advancing its study and practice in health professions education. Method The authors searched five online databases for articles published on spaced learning in health professions education prior to February 2018. Two researchers independently screened articles for eligibility with set inclusion criteria. They extracted and analysed key data using both quantitative and qualitative methods. Results Of the 2972 records retrieved, 120 articles were included in the review. More than 90% of these articles were published in the last 10 years. The definition of spaced learning varied widely and was often not theoretically grounded. Spaced learning was applied in distinct contexts, including online learning, simulation training and classroom settings. There was a large variety of spacing formats, ranging from dispersion of information or practice on a single day, to intervals lasting several months. Generally, spaced learning was implemented in practice or testing phases and rarely during teaching. Conclusions Spaced learning is infrequently and poorly defined in the health professions education literature. We propose a comprehensive definition of spaced learning and emphasise that detailed descriptions of spacing formats are needed in future research to facilitate the operationalisation of spaced learning research and practice in health professions education.

systematic analysis of spaced learning research is complicated by the great diversity in the terms and definitions used in this literature, including 'distributed practice', 'spaced education', and 'retrieval practice.' [8][9][10][11][12] The variety of learning and assessment methods that are referred to as spaced learning further complicate the analysis of its effects. According to definitions used by psychologists, spaced learning should include learning sessions that are spaced over time and include repeated information. 13 Both cumulative testing and simulation training as performed in HPE, for instance, can be considered applications of spaced learning. In addition to the variety of educational activities, spacing formats often differ in terms of their temporality, with some researchers distributing learning sessions over a few days, whereas others use hours, weeks or months. Moreover, it is often unclear if researchers used evidence from empirical research or relied on a theoretical framework to inform their spacing format. Overall, the broad range of terms associated with spaced learning, the multiple definitions and variety of applications used in HPE can hinder the operationalisation of spaced learning.
A comprehensive synthesis of the various definitions and applications of spaced learning in HPE may help identify gaps in knowledge, highlight areas for future research and support a more effective implementation of spaced learning in the HPE curricula.
Therefore, the purpose of this paper was to investigate how spaced learning is defined and applied across HPE contexts.

| ME THODS
We employed a scoping review methodology to examine the definitions and applications of spaced learning in HPE. To execute the review in a rigorous manner, we assembled a research team consisting of co-investigators with in-depth knowledge of HPE (MV, RAH, AT, BWCO and PS), methodological experience (AT and BWCO), 14 and medical library expertise (CP).
We used the methodological framework developed by Arksey and O'Malley, 14 which was later refined by Levac and colleagues. 15 The framework consists of the following six steps: Step 1, identifying the research question; Step 2, identifying relevant articles; Step 3, selecting articles; Step 4, charting the data; Step 5 collating, summarising and reporting the results, and Step 6, consultation. Step 6, consultation, was not conducted as we aimed to study the HPE literature specifically without including additional stakeholders' perspectives on this matter.

| Identifying the research question
Given our goal of identifying key concepts, and applications of spaced learning, we generated a main research question that allows for a broad exploration of spaced learning. The overarching question guiding this scoping review was as follows: 'How is spaced learning defined and applied in HPE?' Accordingly, we sought to answer the following specific research questions: (RQ1A) Which concepts are used to define spaced learning and as-

| Selecting the studies
There was no restriction on year of publication; therefore, all articles published up until 28 February 2018 were screened for eligibility. To be included, articles had to: (a) focus on HPE (eg medicine, nursing, pharmacology), and (b) explicitly name 'spaced learning', or any associated term with a spaced study format. We excluded editorials, commentaries, conference abstracts and books, as well as non-English articles.
Two researchers (MV and RAH) tested the inclusion criteria on a 10% subset of titles. 16,17 A single calibration exercise was sufficient for the team to reach full agreement after inclusion criteria were discussed and clarified. In the abstract screening stage, RAH and MV tested the inclusion criteria using a subset of papers (5%). After reaching full agreement, MV independently screened the remaining abstracts. Two additional calibration exercises were performed with RH independently screening 2.5% of abstracts (n = 34) halfway and again 2.5% (n = 34) at the end of the process to ensure that MV's interpretation of the inclusion criteria was consistent with the original calibration outcome. Disagreements were resolved by discussion. If the focus of the article was unclear based on the title and abstract, the full article was inspected.

| Charting the data
The data charting form was developed by MV and RAH based on the units of analysis included in the research questions (eg definition, theoretical framework, timing of events and setting) using Microsoft ® Excel 2010 (Microsoft Corp., Redmond, WA, USA). They independently extracted data from five full text articles to pilot the form. The usability of the charting form was discussed and minor modifications were made accordingly (ie extraction categories were added and others were removed). For instance, the 'intervention design' category from a previous version of the charting form was merged with the 'timing of events' category in the final version. The process was repeated with an additional five full text articles, followed by discussion, resulting in a final extraction form comprised of the following categories: title; author; publication year; location; terms used for spaced learning; definition by researchers; theoretical framework; population; research method; research design; report of evidence-based spacing; timing of events; topic of learning; type of knowledge; setting; basic sciences/clinical, and learning phase.

| Numerical analyses
We performed a numerical analysis to describe the study characteristics (ie year of publication, location, population, educational content, domain, subject), theoretical frameworks (RQ2A) and spacing formats (RQ3A) included in each paper.

| Thematic analyses
The variety of spaced learning definitions and associated terms (RQ1A) were synthesised using a thematic analysis. Two researchers (MV, RAH) generated a list of open codes from words or phrases in the definitions. Discussion between the two researchers explored relationships between open codes across definitions, which we refer to as concepts. These concepts were then analysed to generate overarching core themes. Drawing from the previously identified core themes as predetermined categories, we used a deductive approach to search for conceptual overlap amongst terms and definitions (RQ1B). Cross-checking of coding strategies and interpretation of data was performed by BWCO.

| Descriptive summary
The database search resulted in a total of 2972 records ( Figure 1

| Definitions of spaced learning
Besides the term 'spaced learning', we found 20 associated terms used to define this concept. Some terms were found in multiple  (14/74). For each core theme, large variation was found amongst definitions, which resulted in a number of sub-themes (see Table 1). For instance, an 'educational activity' was Due to this large variation in definitions, a deductive approach was necessary to study conceptual overlap between terms and this approach was conducted on the core theme level. The recurrent core themes for each of the 21 terms are shown in Table 2. For instance, for the term 'spaced learning' we found five definitions all of which included a notion of a certain educational activity, structure and timing.

| Framing spaced learning
Almost half of the empirical research articles (n = 48, 47%) did not explicitly mention a theoretical framework. In total, nine theoretical frameworks were mentioned in the remaining stud-  Only four studies 50,81,113,114 (4%) focused efforts on spaced learning as a means of teaching, for example, during conventional lectures.

| D ISCUSS I ON
We conducted a scoping review to examine how spaced learning is defined and applied in HPE. Spaced learning appeared relatively new to HPE, with 90% of the articles in our review having been published only in the last 10 years. This is an interesting finding given that the first description of the spacing effect dates back to 1885 and has been a major subject of research in the educational psychology literature since. 120 Our findings indicate that most spaced learning applications in HPE involve online learning, which may explain the later presence of spaced learning in our field.
In light of the increasing popularity of spaced learning in HPE, it is concerning that descriptions of its applications lack the necessary detail to support implementation or replication. Our review showed that in most research spaced learning is poorly defined and almost half of the TA B L E 1 A thematic analysis of definitions of spaced learning and its related terms

| Defining spaced learning
We examined 74 definitions of spaced learning and associated These components should be clearly specified for each study on spaced learning to facilitate comparison and crosstalk between spaced learning researchers in our community.

| Framing spaced learning
There is room for improvement regarding framing of the spaced learning concept as almost half of the articles did not explicitly frame their research using a theoretical framework. This might be related to the diversity and vagueness amongst terms used to define spaced learning, which may have complicated researchers' search for previous empirical research and associated theoretical frameworks.
These findings are illustrative of the general underuse of theory in HPE research. 122,123 Importantly, use of theory can help educators and researchers to better understand existing problems and formulate new research questions.

| Applying spaced learning
Spaced learning is applied broadly in HPE, spanning various health professions, subjects, and educational settings (ie online, simulation and classroom). Exploring the specific details of its applications was rather challenging due to the absence of vital information on used spacing formats such as the number and duration of intervals between educational encounters, the duration of the retention interval, and the number and duration of learning sessions. We emphasise that in future research, spacing formats should be reported in detail to ensure reproducibility and generalisability of the outcomes. 124

| Limitations
Although we attempted to be as thorough as possible, our search was limited to the selected databases, search terms and Englishwritten scholarly articles, which may have excluded relevant articles inadvertently. Furthermore, as a scoping review aims to investigate the nature and extent of the research topic, we did not critically appraise the included studies.

CO N FLI C T S O F I NTE R E S T
None.

E TH I C A L A PPROVA L
Reported as not applicable.