In line with the increased interest in the possible implementations of DI in foreign language teaching, the volume of empirical research on the subject has also been expanding. These studies can be broadly categorised into two groups: one focusing on the effectiveness of DI implementation in the EFL classroom, mostly using experimental research design, and the other one exploring EFL teachers’ perspectives of the approach. Sections 2.1.5.2 and 2.1.5.3 provide an overview of these studies as well as the common themes that emerge from their findings.

 

Aliakbari and Haghighi’s (2014) quasi-experimental study investigated the impact of readiness-based DI on Iranian adolescent and young adult students’ reading comprehension. The study was conducted at a private language institute, where eight classes, including elementary, intermediate and advanced groups, were selected and split into control and treatment groups. The treatment groups received readiness-based DI in the form of flexible grouping and tiered assignments, while the students in the control groups were subjected to traditional, non-differentiated instruction. The authors found that DI significantly improved the reading comprehension in the elementary and intermediate experimental groups, but it did not show a significant difference in the advanced groups.

Another quasi-experimental study focusing on the outcomes of readiness-based differentiated reading instruction was conducted by Magableh and Abdullah (2019). The study involved 55 Jordanian elementary school students, 28 of whom were put into an experimental group that received DI in reading skills development, through the same strategies as in Aliakbari and Haghighi’s (2014) study, i.e., flexible grouping and tiered instruction. The authors found a statistically significant difference between the test results of the experimental group and the control group. Later, Magableh and Abdullah (2021) replicated their study in the secondary school context as well, with the participation of 54 Jordanian students. Similar to their 2019 study, the authors found that the experimental group significantly improved in their reading comprehension compared with the control group.

Differentiated reading skills development was at the centre of a study by El Khdar et al. (2019) as well, although the DI dimension examined was not readiness, as in the previous three studies (Aliakbari & Haghighi, 2014; Magableh & Abdullah, 2019, 2021), but interests. The authors examined the impact of interest-based grouping as a DI strategy on reading skills development in a case study of a Moroccan EFL secondary school class, with data being collected through classroom observations and student interviews. Overall, the study revealed that students’ motivation and engagement increased as a result of interest-based DI.

In Pourdana and Shahpouri Rad’s (2017) quasi-experimental study the focus was on listening skills development, based on readiness-based DI. The study was conducted at a private language school in Iran, with the participation of 46 female students between the ages of 18 and 30. While the control group received non-differentiated listening comprehension tasks, the experimental group completed tiered listening tasks including open-ended, multiple-choice, and true/false questions. Similar to the studies mentioned earlier, the results showed a significant improvement in the listening comprehension of the experimental groups compared with those in the control group.

More recently, in her mixed methods classroom research study conducted with 30 Thai undergraduate university students, Rattanasaeng (2019) examined how a differentiated-flipped approach to speaking skills development affects students’ speaking ability and willingness to communicate (WTC) as well as their perceptions of the differentiated-flipped learning approach. Data were collected with the help of pre- and post-test WTC questionnaires, speaking tests taken at specific points during the 12-week intervention, classroom observation and a questionnaire on students’ perceptions of the intervention taken at the end of the study. Differentiation was based on students’ readiness levels, interest and learning profiles. Results showed a significant improvement in the participants’ speaking skills and WTC and positive perceptions of the differentiated-flipped learning approach, including increased confidence in speaking abilities.

Writing skills were also examined in the context of DI in a small-scale action research study by Argyropoulou and Nina Zafiri (2021). Their research focused on developing the writing skills of a group of eleven Greek upper-primary school students through the use of differentiated teaching materials and a variety of writing genres tailored to the students’ interests, readiness levels, and learning profiles. The authors found a small improvement in the participants’ writing skills following the intervention, although they highlighted that the study faced time constraints and that the instruction was limited to a small cohort of students.

While studies focusing on the development of the four skills abound, research on the effects of differentiated grammar and vocabulary teaching appears to be scarce. One recent study focusing on the former was conducted by Melka and Jatta (2022) in the Ethiopian secondary school context. This quasi-experimental study examined the effects of readiness-based DI on 12th-grade secondary school EFL students’ grammar test scores. The experimental group (n = 23) was subjected to an intervention based on flexible grouping, tiered tasks, and scaffolding, while the control group (n = 24) received non-differentiated instruction. Based on the results of the post-tests, the authors found that the experimental group significantly outperformed the control group.

A summary of the main features of the studies outlined above are presented in Table 1, in the order in which they were discussed. There are four themes emerging from their overview that merit discussion here. Firstly, as has been pointed out earlier, almost all of the studies adopted a (quasi)experimental design to test the effects of DI on a specific language skill, and to a varying degree, all reported on improvements in the investigated skills. However, it is important to interpret these results with caution as issues such as the file drawer problem or publication bias (Rosenthal, 1979) may be at play, i.e., studies with non-significant results could be underrepresented in the published literature. Notwithstanding these considerations, it may be concluded that in many cases there has been a discernible positive effect of DI on student performance.

 

Besides, there are explicit mentions of certain DI principles being used in these studies. Firstly, regarding respectful tasks, several studies implemented tiered instruction to make sure that students at different proficiency levels are provided with appropriately challenging tasks (Argyropoulou & Zafiri, 2021; Magableh & Abdullah, 2019; Melka & Jatta, 2022; Pourdana & Shahpouri Rad, 2017). Secondly, flexible grouping was also used in various studies, with students being placed in dynamic groups based on their proficiency levels or preferences (Aliakbari & Haghighi, 2014; Magableh & Abdullah, 2019; Melka & Jatta, 2022; Pourdana & Shahpouri Rad, 2017; Rattanasaeng, 2019). Thirdly, to a smaller extent, but ongoing assessment also featured in some studies, where teachers adjusted instruction based on continuous feedback from students (Pourdana & Shahpouri Rad, 2017; Rattanasaeng, 2019). However, it is interesting to note that the principle of outcomes-based learning design, which is closely related to the principle of ongoing assessment, was not present either explicitly or implicitly in the reviewed studies, nor was student choice apparent in the interventions examined. This suggests that although certain core principles of DI, such as tiered tasks and flexible grouping, are frequently examined in research on DI in TEFL, other aspects—including outcomes-based learning design and student choice—have received comparatively less scholarly attention.

Finally, it is noteworthy that while student achievement has been extensively researched in the context of differentiated EFL teaching, cognitive or affective variables such as motivation or self-efficacy beliefs have been only sporadically investigated. It would be important to examine how these variables are related to differentiated teaching as both motivation (Piniel & Csizér, 2013) and self-efficacy beliefs (Raoofi et al., 2012) are regarded as important predictor variables leading to the success of foreign language learning.

Bourini’s (2015) mixed methods study investigated the DI-related beliefs and practices of public secondary school EFL teachers in the United Arab Emirates (UAE). The study involved a questionnaire completed by 196 English language teachers working in the UAE, combined with six semi-structured interviews and classroom observations. The statistical analysis of the questionnaire results and the thematic analysis of the qualitative data revealed that teachers acknowledged the importance of responding to learner differences, but many of them had limited understanding of DI and its classroom implementation. Formative assessment, technology, and graphic organisers were identified as the main enablers of DI, while time constraints, inadequate flexibility in the curriculum, and a lack of training in DI emerged as the main challenges of implementation.

Similar to Bourini (2015), Chien’s (2015) study focused on both EFL teachers’ perceptions and practices. Her mixed method study took a primarily qualitative approach with a hint of quantitative inquiry. The author analysed 33 Taiwanese elementary school EFL teachers’ perceptions and practices of DI during an intensive summer course designed to train participants on teaching mixed-level groups. During the course, teachers were asked to complete assignments such as designing and demonstrating activities. The sources of data collection included a questionnaire on teachers’ beliefs of DI, documents (lesson plans, teaching aids, notes) and video recordings of the teachers’ demo lessons. Semi-structured interviews were also conducted to investigate how the teachers constructed their knowledge of DI. Chien’s findings revealed that the participants’ high appreciation of DI was not reflected in their daily classroom practice due their lack of knowledge on the practicalities of implementation and their fears of having to cope with an increased workload.

Tzanni’s (2018) exploratory study sought to provide an overview of the status of DI in TEFL in the Greek context. The author, similar to Chien (2015), investigated teachers’ beliefs and self-reported practices of DI as well as the possible connection between these two aspects. The author took a quantitative approach by asking participants (n = 234) to fill in an online questionnaire in which they had to respond to 42 statements on a 5-point Likert scale. The participants were teaching in a variety of contexts including language institutions, primary and secondary schools, tertiary education, and private tuition. The findings of the study were in perfect concurrence with those of Chien (2015) as they concluded that participants tend to differentiate much less than their otherwise positive beliefs would indicate due to their lack of training on how to implement DI effectively as well as the amount of planning DI demands from them.

In his qualitative investigation, Gülsen (2018) conducted written interviews with 10 Turkish EFL teachers who had been applying DI for at least a year in a Turkish private school with a DI-focused curriculum. Based on a thematic analysis of the replies, the author identified 6 major themes, out of which four pertained to the benefits of DI and two were related to the obstacles of implementation. The most rewarding benefits were identified as heeding learners’ needs, boosting learner confidence, establishing better rapport and promoting involvement and interaction, while the difficulties included the difficulties in implementation and confronting mandatory interventions.

More recently, Granås (2019) conducted an interview study (n = 3) with Norwegian primary and secondary school EFL teachers to examine their practices of DI and the factors influencing their ability to do so. The findings indicated that all participants acknowledge the importance of catering for students’ individual needs, and that they differentiate in their classes to a moderate extent, primarily in content and process. However, time constraints and a lack of competence on how to use DI effectively, especially in mixed-level groups, were identified as the main challenges of implementation.

A mixed methods study by Sougari and Mavroudi (2019) investigated the implementation of DI in Greek primary school EFL classes. Data were gathered through a self-report questionnaire completed by 149 EFL teachers and the classroom observations of 10 EFL teachers. The results revealed a discrepancy between the teachers’ positive views on DI and their actual classroom practices, which largely remained teacher-centred. The study also found a mismatch between the frequency of DI strategies reported by the teachers and what was actually observed in practice, which, as the authors argue, may stem from the misconceptions about the practical application of DI, as well as from time constraints and concerns of potential discipline issues in differentiated teaching environments.

In their questionnaire study, Lombarkia and Guerza (2021) examined EFL students’ and teachers’ attitudes towards DI in the Algerian tertiary context. Data were collected through questionnaires completed by 40 second-year students and 15 teachers from the English department of an Algerian university. The results indicated that both the students and the teachers had a generally positive attitude towards DI. However, in line with the findings of the other studies mentioned earlier, the authors concluded that while teachers are knowledgeable about DI, there seems to be a gap between understanding the principles of this approach and applying it effectively, which highlights the need for training that equips teachers with the necessary knowledge and skills to implement DI.

In a more recent questionnaire study, Rovai and Pfingsthorn (2022) looked at another segment of the EFL teacher population, as they were focusing on pre-service EFL teachers’ beliefs of DI. The instrument included both open-ended and closed-ended questions, with the former examining the participants’ conceptualisations of ‘good’ and ‘bad’ foreign language teachers, and the latter measuring the participants’ quantitative evaluations of existing DI approaches. The study revealed that while the participants expressed general agreement towards the importance of supporting their learners’ individual needs, their knowledge of how to effectively respond to these needs were insufficient.

Tokatlı’s (2022) case study investigated 15 EFL teachers’ perceptions of DI at a private middle school in Turkey. Data were collected through interviews and the analysis of ten lesson plans. The study revealed that teachers primarily differentiate the learning products based on their students’ readiness levels, learning profiles, and interests, and that they see the benefits of DI in enhancing student motivation, engagement, and academic progress. A lack of institutional support and training on DI were identified as the primary challenges of implementation, while collaboration among colleagues emerged as an important enabler that could help teachers overcome these obstacles.

A questionnaire study by Maruf (2023) focused on the relationship between Indonesian EFL teachers’ beliefs, attitudes, and professional development concerning DI, as well as their DI practices. The author used an online questionnaire to collect data from 100 teachers from various schools in Indonesia. The results indicated a strong positive correlation (r = 0.75, p < 0.001) between teachers’ beliefs and practices of DI, as well as between their beliefs of DI and professional development (r = 0.65, p < 0.001), which points towards the importance of professional training in shaping teachers’ beliefs and attitudes towards DI, as well as of the connection of the latter with teachers’ use of the approach.

While the above studies focused on different research contexts and varied greatly in their research methodology, there are certain overarching themes that emerge from their findings. Firstly, the reviewed studies indicate that EFL teachers have varying degrees of knowledge and experience regarding the main principles of DI. Ongoing assessment is recognised as an important element of DI, as teachers in several of the studies reported to be employing formative assessment, self-assessment, and peer feedback to monitor student progress (Bourini, 2015; Granås, 2019; Tokatlı, 2022). However, the principle of outcomes-based learning design was not identified in the reviewed studies, which suggests that predefined learning outcomes—while they may play an organising role in teachers’ instructional planning—are not articulated by teachers as a conscious choice. The importance of flexible grouping was acknowledged in some studies, too (Lombarkia & Guerza, 2021; Tzanni, 2018), as was the use of respectful, appropriately challenging tasks (Bourini, 2015; Chien, 2015; Granås, 2019; Gülsen, 2018). Student choice, although less commonly emphasised, was also mentioned as a way to address learner preferences and interests (Rovai & Pfingsthorn, 2022; Sougari & Mavroudi, 2019; Tokatlı, 2022). These findings indicate that EFL teachers generally hold positive attitudes towards the approach and demonstrate an awareness of the main principles of DI, albeit to varying degrees.

Secondly, results suggest that teachers use DI less frequently than their otherwise positive beliefs would indicate. The mismatch between beliefs and practices can be traced back to various reasons. Firstly, many teachers appear to lack the methodological knowledge for implementing DI effectively (Bourini, 2015; Chien, 2015; Rovai & Pfingsthorn, 2022; Sougari & Mavroudi, 2019; Tokatlı, 2022; Tzanni, 2018), while the additional planning time required by DI was found to be a major challenge too (Bourini, 2015; Gülsen, 2018; Granås, 2019; Lombarkia & Guerza, 2021; Sougari & Mavroudi, 2019; Tzanni, 2018). As highlighted by Gülsen (2018) and Tzanni (2018), DI does not only entail the constant adaptation of the curriculum but presupposes a continuous, formative assessment of students’ needs, which teachers need to be familiarised with. Besides, some teachers find it difficult to multitask in differentiated settings (Gülsen, 2018) and have concerns that DI may lead to discipline issues (Bourini, 2015; Sougari & Mavroudi, 2019). These problems are in accordance with the difficulties identified with DI in general (Blaz, 2016; Willard-Holt, 1994) and call for the exploration of ways EFL teachers could be better assisted in their DI endeavours.

Thirdly, what may strike the reader as interesting is that although the problems of inadequate classroom facilities and lack of materials were hinted at in Chien’s (2015) and Tzanni’s (2018) study, these factors were not deemed to be as significant as one would assume. What is equally reassuring is that except for Tokatlı (2022), none of the authors found problems regarding the schools’ attitude towards DI, which suggests that teachers are generally welcome to experiment with this relatively novel approach. This is a promising pattern as institutional support is an important factor in the successful implementation of DI (Hertberg-Davis & Brighton, 2006; Lewis & Batts, 2005; Tomlinson & Allan, 2000).

Finally, it is also interesting to see what aspects are identified as enablers of DI implementation in TEFL. While out of the studies reviewed, only Bourini (2015) and Tokatlı (2022) focused explicitly on DI enablers as part of their research objectives, recommendations from the other studies also shed light on certain enablers. Without exception, all the studies identified teachers’ competence in DI, especially for mixed-level groups, as a critical enabler, and, closely related to this aspect, the importance of professional development, as well. Technology was also recognised as a potential facilitator of DI practices in some studies (Bourini, 2015; Tzanni, 2018), as well as collaboration among teachers (Granås, 2019; Tokatlı, 2022), which, in line with other authors (Blaz, 2016; De Neve & Devos, 2016; Theisen, 2002; Tomlinson & Allan, 2000), highlights the value of a supportive professional community. Table 2 provides an overview of the main challenges and enablers of DI identified in the reviewed empirical literature on EFL teachers’ beliefs and practices of DI.