The bulk of articles builds upon TDT with other theories. A total of 29 articles out of 42, or 69.048 percent of the total, include other theories in the SDT. The theories integrated with TDT are Bloom’s taxonomy theory, the person–environment interaction model, the theory of mediated learning experience (MLE), Computer-Based Scaffolds, the community of inquiry, rational analysis of mobile education (FRAME), self-regulated learning (SRL), the social cognitive theoretical framework, computer self-efficacy, cognitive load theory, activity theory, sociocultural theory, the social science theory, the cultural–historical theory, the activity theory, the transactional distance theory, the transactional control theory, shaping dwellings, and the stigmergy. The significant number of theories in TDT may be due to its strong explanatory power. As a consequence, TDT was combined with other theories and models to improve the explanatory capacity of such theories and models. This is not a new position or approach since it has existed in the past. The creator of TDT proposed and recommended its inclusion [12].

Moore’s transactional distance theory (TDT) is a valuable paradigm for studying remote education [1,2,10,11,12,13,23]. TDT describes and quantifies the instructor–student learning interaction in distance education [24]. High TD between instructor and pupils may cause isolation, low motivation, and disengagement [2,23]. Moore identifies three TDT concepts: (1) structure, (2) interaction (or dialogue), and (3) learner autonomy [7].

The structure represents the interaction between the teacher, students, and technology [25,26,27]. Autonomy is the degree of structure needed; promoting interaction and fostering learner autonomy is difficult. The more structure and the less interaction, the more learner autonomy is necessary [7]. The dialogue that occurs as part of the learning process [10] assists students in conceptualizing [28]. Successful TD settings rely on the instructor delivering interaction and “appropriately” arranged learning resources. Greater, quicker, and more involved connection reduces psychological isolation [28,29]. Effective online learning requires well-structured information, the latest technology, and more interactivity [30].

Researchers categorized the article based on the TDT factors used in their studies. Seven of the forty-two evaluated studies, or 16.667 percent, employed TDT factors (structure, dialogue, and learner autonomy) without any integration [15,31,32,33,34,35,36] based on an examination of the reviewed articles. In addition, 35 of the forty-two examined articles, or 83.333 percent, incorporated other aspects into TD theory.

One study [37] has integrated self-regulated learning (SRL) with TDT in their study. Moreover, the integration between TDT and Bloom’s taxonomy theory (BTT) was the theoretical framework of [38]. In one study [39], TDT was integrated with a person–environment interaction model. One study combined the theory of mediated learning experiences (MLE) with TDT [40]. In addition, the integration between problem-based learning, computer-based scaffolds, and TDT was found in one study [35]. At the same time, one study has integrated TDT and the community of inquiry (CoI) [41]. One study integrated the rational analysis of mobile education (FRAME) with TDT [42]. Another study combined the social cognitive theoretical framework with TDT [43]. Computer self-efficacy with TDT was integrated into [19]. The integration between multiple theories, cognitive load theory, activity theory, sociocultural theory, and TDT was found in [20]. The integration of TD, social science theory, cultural–historical theory, and activity theory is discussed in [13]. Finally, transactional distance, transactional control, shaping dwellings, and stigmergy were found in [38]. For example, ref. [44] studied only two TDT factors: structure and dialogue, leaving learning autonomy alone; ref. [45] examined only dialogue from the TDT (student–content, student–interface, student–instructor, and student–student interactions).

Understanding the kind of sample is essential for justifying the selection of samples for future research and understanding the present knowledge gap in the context of TDT research and distance learning. In light of the theory’s focus on human happiness as its endpoint, TDT studies are almost exclusively conducted with people in mind [9,16,38,41,46,47,48,49]. Based on the examination of the samples used in the selected publications, researchers can confidently say that the vast majority of samples for SDT studies of online education consist primarily of students. Thirty-five total samples were obtained, with 83.333 percent coming from students. In just 4.76 percent of the studies, lecturer samples existed (n = 2). In addition, 4.762% (n = 2) of the studies employed surveys of in-service teachers.

Moreover, both students and faculty administrators were surveyed in 2.381% (n = 1), and both students and module coordinators were surveyed in 2.381% (n = 1), as well as students and lecturers. It is possible that combining student and teacher samples is an effort to understand the motives behind the whole distance learning process from the viewpoints of both the information receiver and the instructor. In addition, under the guidance of faculty administrators, students will work closely with module coordinators to develop an in-depth comprehension of the defining features of course design concepts based on Moore’s TD theory.

In the early days of TDT’s development, quantitative research methodologies were employed to deduce the connections between TDT elements such as course structure, discourse, and student agency. Twenty-one papers, or 50% (n = 21), used quantitative research methodologies. Tactical decision-making (TDM) and the continuing online learning initiative [19,20,21,31,36,38,39,40,44,46,47,48,49,50,51,52,53] continue to use quantitative methodologies.

Quantitative methods, however, have seen a rise in favor as well. Only 19.048 percent (n = 8) of articles were published using qualitative approaches such as [54] open-ended interviews, bulletin board peers’ discussion logs, research writing assignments, video and audio transcripts and observations notes [42], focus group interviews [55], case studies [56], and content analysis [13,14,15]. The most common qualitative methods used were case studies.

In addition, 30.952% (n = 13) of the examined publications showed that mixed-method techniques were more prevalent than qualitative ones. A questionnaire and a virtual, semi-structured interview were the most commonly used mixed methods. Other methods included the following: [45] a questionnaire and focus group interview, ref. [57] questionnaire and interview, [16] artificial intelligence sentiment analysis, [17] questionnaires and in-depth interviews, ref. [18] questionnaires and semi-structured interviews, [35] questionnaire and rubric, [58] face-to-face, open-ended interviews, bulletin board discussion logs, and online assessment projects, [41] surveys, instructor journals, and learning activities, [17] a questionnaire and case design, ref. [43] SRL activities, survey answer analysis, and journal reflection, and [59] content analysis and questionnaire.

The study on TDT in distance learning within the setting of universities is geographically diversified. Hence, there is no particular emphasis on places. There are, nevertheless, clear indications of high scientific activity in the US. A total of 18 articles [13,15,19,20,33,35,37,40,41,42,43,46,48,51,53,54,58] (42.857%) were carried out and published in the US. There have been just four investigations undertaken in Turkey [18,45,49,57], three in Malaysia [21,38,55], two in New Zealand [34,56], two in China [39,52], and two more in India [31,60]. Eleven papers were published globally in the interim, accounting for 25.3% of the articles examined. For example, the United Kingdom (n = 1), Thailand (n = 1), Sweden (n = 1), Hong Kong (n = 1), Greece (n = 1), South Africa (n = 1), Palestine (n = 1), Malawi (n = 1), the Philippines (n = 1), and Israel (n = 1), as well as a global study (n = 1). Therefore, nothing is known about TDT in African institutions’ distance education programs.

Based on the analysis of the evaluated articles, the most common recommendation was that, firstly, the course design or structure must be based on theories and preceding literature to integrate distance learning [37,38,39,40,41,47,48,49]. Secondly, instructors have a crucial role in distance learning contexts by providing support and encouragement. Moreover, reasonable distance education tutors and advisors create a student-centered learning environment, care about students, and have subject understanding and basic technical abilities [50,51,52,53,54]. Another recommendation was that TDT is updated to reflect the use of synchronous technologies for remote learning, especially its definition and perspective on structural aspects and how synchrony impacts learner autonomy [56]. Finally, TDT promotes and facilitates distance learning. Instructional designers learn about distance learning and how to use technology in teaching and learning [15].