Mapping Lesson Study as a research field: a bibliometric review

Lesson Study is a collaborative and systematic approach to teacher professional development where teachers together plan, teach and observe a lesson, then reflect, as a group, on what they have learnt about teaching and student learning (Seleznyov, 2018). It has been part of the Japanese school system for more than 100 years (Ishii, 2017) but received international attention after Stigler and Hiebert’s book The Teaching Gap, published in 1999 and has since gradually been introduced and developed in a variety of contexts around the world (Lewis and Lee, 2017; Xu and Pedder, 2015). The increasing international interest is demonstrated by the growth in the number of countries participating in the annual conference held by the World Association of Lesson Studies (WALS), from 15 countries in 2007 to 43 in 2019 (Fang and Wang, 2020). The international community of educational researchers has also become increasingly interested in Lesson Study as a research field; the launch of the International Journal for Lesson and Learning Studies in 2012 is one standout milestone in the growth of the field. Our purpose in this bibliometric review is to provide an overview of development in the Lesson Study research field for the period 2002–2023, ultimately comparing it with development in the wider field of educational research.

The accumulation of knowledge is an essential condition for the development of a scientific field (Paré et al., 2015) and literature reviews can provide illumination of that field for the purpose of research. Bibliometrics differs from other types of reviews by its use of quantitative methods and specialised software suited for identifying and mapping development and trends within specific research disciplines. It is used to measure and analyse published research literature in aggregate, including metadata such as abstracts, keywords and citations (Roemer and Borchardt, 2015). It can summarise large quantities of bibliometric data, making possible the analysis of fields, topics, people or organisations (Donthu et al., 2021). Different types of software are used to perform bibliometric analysis and the data used typically come from scientific databases, with Scopus and Web of Science (WoS) the most prominent and reliable sources of bibliographic data (Pranckutè, 2021). The purposes of bibliometric studies can be to measure citation impact, science development, cooperation and different types of rankings and to present intellectual structures and emerging trends in a research field (Donthu et al., 2021).

The general growth in scientific publications and development in publication patterns are important considerations in bibliometric analyses. The overall growth is described as exponential (Bornmann and Mutz, 2015), with factors contributing are the increase in the sheer number of journals as well as articles per journal (Fire and Guestrin, 2019). Regarding changes in publication patterns, there have been tendencies towards an increased number of authors of academic papers and a lengthening of reference lists (Fire and Guestrin, 2019). Some of these changes had been attributed to increasing competitiveness in the academic world (Fire and Guestrin, 2019; Wakeling et al., 2019). The introduction of online-only mega-journals like Plos One and Scientific Reports is another identifiable factor (Thelwall and Sud, 2022). Mega-journals are generally described as those having a broad scope and large output, with funding provided by publishing fees paid by authors prior to publication, and perhaps most importantly, offering a simplified review process (Björk, 2018; Wakeling et al., 2019). They also have a shorter time between submission and acceptance, with current averages of 188 days for Plos One and 140 days for Scientific Reports. There is, however, a question as to what extent mega-journals have affected the field of educational research, as these journals seem to prioritise disciplines like medicine and natural sciences (Wakeling et al., 2019).

Of the variety of reviews conducted within Lesson Study research, one central theme is the effect on student or teacher learning and development (Cheung and Wong, 2014; Willems and Van den Bossche, 2019). The geographical spread, and thus translations of Lesson Study into various cultures and school systems, has, according to Stigler and Hiebert (2016), led to the need for more explicit theorising, ensuring adaptation to local contexts without losing the core ingredients. In line with this, Seleznyov (2018) published a review that focused on the challenges that occur when Lesson Study is translated into different contexts, first reviewing literature to find “critical components” – features identified as crucial to the success of Lesson Study, then reviewing studies taking place outside Japan, ultimately finding that one or more of the critical components of Lesson Study were missing from a large proportion of the studies. As an answer to this challenge, Seleznyov et al. (2021) published a study exploring adaptations of Lesson Study within international contexts and the relevance of national culture in local adaptations.

In addition to these, reviews have explored, for example, the use of Lesson Study in teacher education (Baumfield et al., 2022; Larssen et al., 2018), in special needs and inclusive education contexts (Norwich et al., 2021) and in higher education (Hervas, 2021). One comprehensive literature review was carried out by Xu and Pedder in 2015 and focused on Lesson Study research from 1999 to 2013. The 67 articles included in their review were analysed with respect to different themes. The dominant countries were the USA (32), Hong Kong (six) and the UK (five). The largest category of research themes was focused on benefits and constraints for teachers’ professional development. Another review by Yoshida and colleagues from 2021 categorised Lesson Study-research and examined how research within these categories was distributed across different regions. They found that Asia, as a continent, had the greatest number of publications, with professional development and pedagogical content knowledge the categories most represented.

Reviews like these offer valuable insights into the accumulated knowledge in the Lesson Study research field. Bibliometrics, however, provides insights that are difficult to obtain through traditional reviews alone. The aim of our study is to contribute to the field through a bibliometric review of Lesson Study, consisting of quantitative mapping of peer-reviewed research literature published in English.

1. To map the growth, productivity and most influential actors in the research field of Lesson Study.

2. To map the development of the research field of Lesson Study in terms of dominant research themes and institutions.

3. To compare development in the research field of Lesson Study with the development in educational research in general.

The first step was to design the search in such a way that the set of results yielded would be large enough to support a bibliometric review and yet precise enough to remain within the field. Use of a single database is recommended, as registrations between databases can differ, thus increasing the risk of errors (Donthu et al., 2021). We chose ISI Web of Science, as it provides wide coverage of research literature and is known to index research in ways amenable to bibliometrics (Pranckutè, 2021; Roemer and Borchardt, 2015).

In bibliometric analysis, the quality of the data set is crucial. In developing our dataset, we systematically employed different search strategies, manually checking the lists produced. We wanted to achieve the widest possible coverage but also to avoid irrelevant articles – what Walters (2016) describes as a trade-off between recall and precision. As an example, we searched for “lesson study” in abstracts but found that it was often mentioned simply as an example, without being within the scope of the article. Discovering that some central articles were missing, we included “lesson study” as part of the title in the search, knowing that some journals did not index keywords.

This process led us to the following search of peer-reviewed articles in English: First two separate searches: author keyword “lesson study” (N = 392) and title “lesson study” (N = 369). These two searches were then combined with OR as a Boolean operator, resulting in 446 unique articles (duplicates removed) published between 2002 and 2023.

To be able to compare growth and development in the Lesson Study field with wider changes in academic publications in general, we performed an additional search in WoS concerning the general field of educational research for the same period (2002–2023). Our search was for education and educational research and articles and review articles, yielding 290,393 articles.

The final searches were conducted on 17 September 2024.

The Lesson Study dataset (N = 446) was then analysed according to our research aims, leading to the mapping of growth, productivity, main influences, development in dominant research themes and development in dominant institutions in the Lesson Study research field.

Our first step was to analyse the dataset regarding growth, productivity and the most influential journals, authors and publications. To accomplish this, we used tools for frequency counts and citation data in WoS. We extracted publications per year, most cited articles and the journals with the highest number of publications and number of citations in the journals from the dataset.

We then imported the dataset into the VOSviewer software, a tool for bibliometric analysis which extracts and makes graphical representations of information – in this case, from the selected articles. VOSviewer displays results of analysis in graphical maps, where the units of analysis (e.g. authors or keywords) are represented by circular nodes and relation between nodes are represented by lines. The size of nodes represents frequency, and the thickness of lines represents the strength of the relationship between nodes. The position of nodes reflects interrelatedness; the closer two nodes are, the stronger their relation. The colour of nodes represents average publication year (van Eck and Waltman, 2022). We used VOSviewer to map development in the Lesson Study research field through two different analyses. First, we performed a bibliographic coupling analysis of organisations to explore development in what universities are represented and how they are related to one other. This analysis compares reference lists in publications, with the assumption that the greater the degree to which publications from two organisations share references, the more closely related they are in terms of theoretical perspectives and methodological approaches (Wallin, 2005). Afterwards, we conducted a co-occurrence analysis of author keywords to get an overview of research themes (Donthu et al., 2021). Both these analyses are visualised through VOSviewer to show the average publication year.

Finally, to facilitate the possibility of comparing development in Lesson Study research with development in educational research in general, we analysed the dataset concerning educational research (N = 290,393), using tools for analysis inherent in WoS. We obtained information about the number of publications per year, dominant journals and countries.

The growth in number of publications which include Lesson Study research is presented in Figure 1. From the first indexed article in 2002, there was very limited growth for the next 10 years. Then we see a small increase in publications, most likely triggered by the establishment of the International Journal for Lesson and Learning Studies, the first edition of which was published in 2012.

We found an increase in number of publications in 2018 and again in 2019. The growth is significant to the degree that 76% (N = 339) of the articles in our dataset were published during the last five years (2019–2023). The growth appears to have levelled off in recent years, peaking in 2020. In addition to the growth in number of publications, there has also been a substantial increase in geographical spread during the same period. From 2002 to 2018, we found articles from 22 different countries and between 2019 and 2023, we found articles from 55 countries. All continents are represented in our dataset, with the USA, the UK and China the most productive countries, with 97, 48 and 40 articles, respectively.

The 446 articles in our dataset were published across 149 different journals, the most influential of them listed in Table 1. About a quarter of the articles (27.8%) were published in The International Journal for Lesson and Learning Studies, and the greatest number of citations are found in Teaching and Teacher Education; those metrics identify these two journals as the most influential in the field.

Most frequent on the list are journals focusing on teaching and teacher education, educational research, professional development and mathematics teaching, though journals covering literacy, science education, physical education and bilingual education are present as well.

In Table 2, the most influential articles in the dataset are listed, ranked by number of citations. Except for Dudley (2013) who is European, the most cited articles were published during the first decade and have either American or Asian authorship. As the recent growth in publications has been so large, we also checked for the most cited articles from the last five years. These were Vermunt et al. (2019), Coenders and Verhoef (2019) and Dudley et al. (2019); all from European countries.

Looking into the most cited authors within the Lesson Study dataset, listed in Table 3, we see that Fernandez is cited almost twice as often as the next author on the list. American authors dominate as the most cited, together with Pang, who is associated with Learning Study research. Again, Dudley is the only European author on the list.

The first analysis with VOSviewer software explored development and relations between dominant institutions. The VOSviewer map in Figure 2 is the result of a bibliographic coupling analysis, showing universities represented with a minimum of five publications (N = 23). Bibliographic coupling is based on shared references in the different publications. Each circle represents a university, and the size of circles represent number of publications, while the proximity of the circles represents their degree of shared references; the closer universities are to each other on the map, the more they tend to cite the same publications. The colour of circles represents the average publication year for articles. Because of the much larger number of articles published during the last five years, the map in Figure 2 accounts for the years from 2016 through 2022. The universities with the most publications from 2002 to 2023 are Nanyang Technological University, the University of Lisbon, the University of Stavanger (Norway) and Vrije Universiteit Amsterdam.

The map in Figure 2 shows a cluster of Dutch universities at the right (VU Amsterdam, Groningen, Windesheim and Twente). Their proximity to one another and the thickness of lines between them indicate a high number of shared references, with their colour indicating that their publications are of relatively recent date. This strong grouping could imply these universities differ in their research interest, as compared with the rest of the field, an observation that led us to examine the keywords used in the Dutch articles. The most used keywords align with the rest of the field, but in addition we found terms related to leadership, self-efficacy and inclusive and adaptive teaching. At the top of the map in Figure 2 is a cluster of three universities: Shanghai, Middle Tennessee and Beijing. Their proximity also indicates a high number of shared references, a fact we found explained by the large proportion of articles published in collaboration between these universities. The most recent average publication year is represented by universities from Shanghai, Beijing, Middle Tennessee and Lisbon; the earliest are represented by Hong Kong, Tokyo, Copenhagen and Leicester. As the USA is the country represented with the most publications, one would expect them to have a higher representation in this map; the explanation for this is that the articles are published from many different universities.

The second analysis with VOSviewer software was to explore development and relations between author keywords. The VOSviewer map in Figure 3 is the result of a co-occurrence analysis of author keywords, showing those used in a minimum of five publications. In this analysis we chose to manually remove some keywords to get a clearer picture of research themes. The removed keywords are Lesson Study, terms describing research methods and terms referencing geographical placement. Each node in the map represents a keyword and the size of the nodes represents the number of usages across all publications in the dataset. As with Figure 2, the proximity of nodes to one another represents their degree of shared use and their colour represents the average publication year where each keyword was used.

The map in Figure 3 shows some central themes in the articles published from 2002 to 2023. Most prominent are different terms regarding teachers’ professional development and teacher education. The darker colours indicate that these themes, together with terms regarding mathematics, are dominant in the earlier publications. Circles with lighter colour indicate more recent research themes; these include collaborative learning, higher education, inclusive education and physical education. Online Lesson Study is also one recent theme, suggesting a rising interest in the use of technology as a tool. The darkest nodes in the map are those regarding Learning Study (variation theory being part of a Learning Study), with the colours indicating that these publications have the earliest average publication year. Because of the much larger number of articles published during the last five years, the average publication year is between 2016 and 2022 in this map as well.

To be able to compare Lesson Study research with general educational research, we first extracted statistics showing growth in all publications related to education per year from WoS during the same period (2002–2023, N = 290,393); these data are presented in Figure 4. We see a growth pattern similar to that for Lesson and Learning Study (Figure 1), with a sharp increase in 2019 and a majority of articles published during the last five years.

In addition, we checked which journals had the most publications. The three top journals were BMC Medical Education with 5,413 publications, Computers and Education with 3,678 and Education Sciences with 3,550. BMC Medical Education was represented with 948 articles for the year 2023 alone. All three of these journals demonstrate characteristics of mega-journals, with simplified review processes (Björk, 2018; Wakeling et al., 2019). Additionally, they are all relatively new, with a large number of articles published. The journal with the fourth largest number of publications at 3,021, was Teaching and Teacher Education, a well-established traditional academic journal. It is worth mentioning the large proportion of journals with medicine, science or technology in the titles.

Looking at what countries dominate the field of general educational research, the top three are all English-speaking. The USA had most articles published by far, with 88,285; the UK was second with 25,541 and Australia was third, with 20,517.

Our aim for this bibliometric review was to map the growth, productivity, main influences and dominant institutions and research themes in the Lesson Study research field. In addition, we have investigated educational research in general during the same period for the sake of comparison. Based on analysis of 446 peer-reviewed articles published in English between 2002 and 2023 and indexed in WoS, this review provides an overview of the evolving field of Lesson Study research. The review shows the development over the more than 20 years where Lesson Study has been part of the international research community, demonstrating that there has been significant growth in terms of quantity, geographical spread and topics, and that although Lesson Study is still young as a research field, it seems to have become well established.

Our first aim for this review was to map the growth, productivity and most influential actors in the research field of Lesson Study. Regarding growth, there has been a significant increase in the number of articles published per year during the 21 years covered in this study. This tendency was described by Xu and Pedder (2015) and seems to have accelerated, especially over the last five years, a period which represents as much as 76% of all publications in our dataset. However, the curve has levelled out after a peak in 2020, showing a small decrease in publications. While the authors suspect the decrease is not the result of fading interest but is instead attributable to reduced research activity during the COVID-19 pandemic, as shown in other studies (e.g. Lokhtina et al., 2022), it is too early to determine this with any certainty.

One factor that has affected the growth of publications is the increasing number of countries represented. During the last five years covered in our review, the number of countries has more than doubled compared to the first 15. This is supported by Fang and Wang (2020), who found a rise in countries participating in the WALS conference, from 15 countries in 2007 to 43 in 2019, an increase that affirms a widening geographical interest in Lesson Study. The geographical diversity of published research in our study demonstrates that Lesson Study has grown into a global phenomenon. Regarding geographical dominance, the USA, the UK and China are the leading countries with respect to the number of publications and citations. While one might expect Japan to have a larger representation in our dataset, because we intentionally limited our search to articles published in English, we may well have excluded significant non-English literature in the field. Interestingly, the analysis from the last five years seems to show a turn in dominance towards European countries, where besides the UK, the Netherlands in particular has become an important influence in the field. The growth in European publications aligns with Yoshida et al. (2021), who found that studies focusing on Lesson Study in Europe have been increasing since 2010.

Our second aim for this review was to map the development of the research field of Lesson Study in terms of dominant research themes and institutions. As in both Yoshida et al. (2021) and Xu and Pedder’s (2015) reviews, we found teacher professional development the most frequent theme. But we also found a growth in research themes, represented by author keywords in the articles. The keywords dominant in the earlier articles were related to teacher professional development and teacher education, with mathematics and science as dominant subjects. Xu and Pedder (2015) found that research on Lesson Study was mainly carried out in primary and secondary schools and teacher education settings, with mathematics and science teachers most represented. In our dataset, we found that higher education has become a frequently used keyword, implying an expansion of educational setting. We still find a strong focus on mathematics and science, but physical education and inclusive education are also now represented among the keywords most used, indicating that the focus of research is widening. The variety in aim and scope of journals publishing research on Lesson Study confirms the growth in research themes.

Learning Study is visible as an important influence in our findings. Pang and Marton are on the list of most cited articles and Learning Study and variation theory are among the frequently used keywords. However, it seems that Learning Study research is still mainly limited to Hong Kong and Sweden, not having the same geographical growth as Lesson Study. The difference in average publication year confirms this, with Learning Study publications being relatively older.

Turning to our last research aim: to compare development in the research of Lesson Study with the development in educational research in general, where we found that growth patterns within Lesson Study research aligns with the growth patterns in educational research. The general educational research field seems to be influenced by the described development in academic publication patterns (Fire and Guestrin, 2019; Thelwall and Sud, 2022). The two top journals are mega-journals with medical and computer education as scopes and the list of journals is strongly dominated by themes related to technology and medicine. These are themes rather peripheral in traditional educational research, but central among the mega-journals (Wakeling et al., 2019). We cannot say anything conclusive about how much of the growth in educational research is due to mega-journals but from our findings it seems to be a large proportion. Lesson Study research does not seem to be influenced by recent trends; its research is still by and large published in traditional journals with regular peer-review processes, so its growth cannot be attributed to the introduction of mega-journals.

While bibliometrics can provide useful tools for mapping the literature in a scientific field, it does have some limitations. We chose to perform our searches in the WoS database and restricted them to peer-reviewed articles published in English. This meant that other types of publications, and those published in other languages were excluded. Although this is a noteworthy limitation, we consider the quantitative approach used in this bibliometric review to be a valuable contribution that provides additional insights to the field. Based on the 446 publications identified, our study has provided an overview of research on Lesson Study during the 21 years since the first indexed publication in 2002. We have mapped growth and developmental trends in terms of geographical and thematic spread and dominant actors in terms of authors, journals and institutions. Using quantitative methods for analysing research literature as we have done in this study allows for wide coverage and the possibility of building a global picture of Lesson Study research.

Performing research like the bibliometric analysis presented herein has given rise to reflections on possible further inquiries.

Given the substantial growth in the volume of publications, research themes and geographical reach, our findings strongly support the recommendations of Stigler and Hiebert (2016) and Seleznyov (2018) regarding the need for more explicit theorisation and discussion of the core features that should be preserved. While the global expansion of Lesson Study has been largely driven by the high quality of Japanese education, Ishii (2017) warns against mere replication of its form without a deeper contextual understanding. Seleznyov (2018) further notes that several key elements that distinguish Lesson Study as a research process were often absent in studies from outside Japan, thereby diminishing its potential as a form of practical research for teachers. This raises the question of how far Lesson Study can diverge from its original structure while still retaining its core identity. This might be both the strength and vulnerability of Lesson Study – and one of the reasons for its popularity: it is easily adapted to local contexts.

It will also be interesting to follow further developments within the Lesson Study research field, particularly with an eye to if and how the field may be influenced by recent publication trends.