Introduction

The advancement of technology embarks a variety of digital media development in digital natives’ society. In tandem with that situation, the development of digital media also has been integrated with education field namely digital media learning. The innovation growing rapidly resulted the emergence of digital textbook development in education arena. With the arrival of the multiple types of format and content easily access across different media and devices lead to various reading experience and expectations. This introduces a new level of publishing process and workflow. Thus, the publishing industry landscape had changed significantly in terms of the publishing content workflow [1,2,3,4,5]. Besides, major changes had occurred encompass upstream phase (authoring, design, editing, layout), production phase (multimedia production) and downstream phase (distribution) [6].

The usage of devise among teenagers are increasing day by day. This scenario occurred around the world including Malaysia. According to the National Cybersecurity Awareness Baseline Study 2016, it was found that, 75% of Malaysian secondary students did an online activity everyday by interacting social networking, surfing YouTube and do schoolwork via smartphone. Based on the figure, digital textbook product should take advantage with the tendency of the teenagers’ behavior. The finding also aligned with the international study which found 54% of e-book reader like to use smartphone when reading the e-book [7]. The percentage indicated 24% increase compared to the same study in 2012 [8, 9]. Therefore, the digital textbook design should lead a positive experience to attract them towards digital textbook.

Since the usage of digital textbook involve the screen reading, the reading behavior of the user will be different compared to the printed textbook [10,11,12,13]. To overcome the challenges, usability elements play a key role at the design phase of digital textbook interface. This argument aligned with the finding from the previous studies whereby the easy access to the digital textbook interface is the main criteria of digital textbook design [14,15,16]. Despite, overwhelming research and standards pertaining usability, there is still a lack of efforts to systematically review the concept of usability elements in the digital textbook development context.

Towards a Systematic Review Framework on Usability Element in Digital Textbook Development

Based on that issues, a systematic literature review of usability element in digital textbook development has been undertaken to reflect a diligent action to find the most suitable concept of usability that can be absorbed at the design phase. The selection of systematic review approach for this study is because the identification, selection, collection and analyzing data critically defined from systematically formulated question [17].

To perform a relevant systematic review, this study was guided by the main research questions:

  • What are the dominant usability factors and usability criteria has been used in the previous study?

  • What are the differences between the usability factors compared to the usability criteria or usability properties?

  • What are the dominant of usability properties that has been adopted in application product and digital textbook?

  • How does the concept of usability factor and usability properties influence the interface design of digital textbook?

A special focus to the usability factor and criteria because the elements can influence the satisfaction and user experience to a specific product including digital textbook especially when designing and developing interface [18,19,20,21,22].

Therefore, this study tries to analysis the existing literature pertaining to the investigation on usability elements encompass usability factor and usability criteria on digital textbook perspective. There are three main segments of this study to realize this, namely the first section that explains the purpose of conducting a systematic review. The second chapter uses the method of throwing out the methodology part using the PRISMA Statement (Preferred Reporting Items Systematic Reviews and Meta-Analysis). While the third section systematically reviews and synthesizes the scientific literature to identify, select and assess suitable research on usability elements of digital textbooks. The last section proposes for future studies the possible direction and priorities.

Methodology

This paper is organized using the Strategy for Systemic Reviews and Meta-Analysis of Preferred Reporting Items (PRISMA) to ensure that this study offers a neutral output composed of data on compilation and evaluation [23]. Scopus and the Scientific Resource Web were used for systemic assessment, criteria for admissibility and exclusion, process assessment (identification, screening, eligibility) and abstraction and analysis of data. According to several scholars, there are three advantages by using the PRISMA technique, namely (1) defining clear study questions that allow systemic study, (2) identifying inclusion and exclusion criteria and (3) it attempts to examine large database of scientific literature in a defined time [17, 24].

Resources

The evaluation is carried out in two primary databases of journals, Scopus and the Web of Science (WoS). WoS is a powerful database of over 33,000 publications from 256 areas, including surveys of computer data systems, behavior and data technology, digital publishing, and interactive learning studies. It includes extensive background records and citations for over 100 years, generated and categorized by Clarivate Analytics as three distinct measures. Scopus is the second database to be used in the assessment. With over 22,800 journals released worldwide by 5000 publishers, it is one of the largest abstracts and citation bases of peer reviews. Scopus includes a range of areas including science, social science, computer science, and communication with human computers.

Eligibility and Exclusion Criteria

There has been a strict action to guarantee that only quality paper is chosen. A few criteria for eligibility and exclusion have been created. First, the only empirical article journal is chosen, while all are excluded from the review essay, book series, book, chapter, and conference. However, one article (full paper) from proceedings of the 13th International Conference on Entertainment Computing, ICEC 2014, held in Sydney, Australia, in October 2013 has been selected due to limitation of identification of usability aspect in empirical research. Secondly, efforts to search are focused solely on English-language documents to avoid confusion and transactional difficulties. Third, the search was not limited by the year of publishing. Fourth, the paper selected only the peer-reviewed journals. Lastly, aligned with the objective of this paper to focus on usability element of digital textbook, only articles within the scope are selected (see Table 1).

Table 1 The inclusion and exclusion criteria
Full size table

Process of Systematic Review

There were four stages in the systematic assessment technique. The evaluation was done in End-2018. In the first phase, search keywords were acknowledged. Similar keywords linked to electronic textbook and usability have been used based on prior research and thesauruses (Table 2). Five duplicated documents have been removed at this point after thorough testing.

Table 2 The search string used for the systematic review process
Full size table

The second stage was screening. Of 213 articles eligible for evaluation, a total of 133 articles have been removed. At this stage. Eligibility is the third stage, providing complete access to articles. After thorough examination, a total of 62 papers, some of which were not digitally concentrated on a book, were not empirical articles and were not concentrated on recognition and adoption. In the last stage of the assessment, a total of 17 articles were used for qualitative analysis (Fig. 1). In 2019, one paper was released, two papers were released in 2018, two papers were released in 2017, two papers were released in 2016, three papers were released in 2015, four papers were published in 2014, two papers were released in 2013 and two papers were released in 2012.

Fig. 1
figure 1

(adapted from Moher et al. [114])

The flow diagram of the study

Full size image

Analysis of Data and Abstraction

The other papers were assessed and analyzed. Specific studies focused on answering the formulated questions as stated in “Towards a Systematic Review Framework on Usability Element in Digital Textbook Development” section. The information was first obtained through the abstracts, then the full article (in-depth), to determine usability factor and criteria for usability. This article also examined the research technique and the study finding. Using content analysis to define the sort of usability related to digital textbook usability research, qualitative assessment was implemented. Usability factor and usability criteria were then structured by the writers via table form.

Result

The review focuses, as stated above, a total of 17 papers, on the basis usability elements of digital textbook studies. Attention was given to the previous research concerning usability factor and usability criteria since these both form a usability element that need to be equipped when designing a digital textbook interface. A better usability element embedded in digital textbook interface is very important due to potential influence students’ satisfaction and strengthening the effectiveness of using digital textbook. Thus, the potential can transform to the effective adoption of digital textbook usage among the students [25]. Moreover, 28 types of usability factors have been used based on the review paper. In addition, more than 40 types of usability criteria have been applied in the prior research. The findings provide a thorough assessment of usability conceptual used for digital textbook usability studies.

The Analysis of Journals

As part of the systematic search, three hundred and twenty-seven published studies were recognized, including a final set of 17 research for qualitative synthesis (information for each research are provided in Table 4). The characteristics and characteristics (research design, samples, and theory used and study finding) of each study along with sample characteristics are provided in Table 4. According to eligibility requirements, 213 documents were recognized. In the Fig. 1, Each box defines the amount of documents (ni) associated results. Seventy-eight papers have been evaluated for eligibility, offering a foundation for characterizing digital textbook usability research. However, it found that only 17 articles eligible to answer the research question. From the analysis of journal types from 2012 to 2019, it was found that, 14 types of journals have been published the article related to the usability studies. The title of journal and number of article that has been published as in Table 3.

Table 3 Title of journal and number of publication
Full size table

The Trend of Research Characteristic

The research characteristic was analyzed to identify the trend of usability conceptual has been used in the usability studies related to digital textbook context. The analyzing involved the aspect of research design, sample selection, data collection method, theory used and the usability factor that has been adopted in the study (refer Table 4). Overall, the survey design was the dominant research method (10 articles), with four studies using either a log file (1 article; i.e. [26]) or interviews (3 articles). Most survey research was cross-sectional (6 articles). Three studies combined individual surveys with interviews. While for the selection of sample, most of the samples for digital textbook usability studies were from school students and university students. In terms of the geographic region, usability research was dominated by Taiwan (4 articles) with a few studies in Asia, such as Korea, Malaysia (2 articles) and a few studies in western country, Canada (2 articles), USA (3 articles) and Greece (1 article).

Table 4 The summary of finding
Full size table

Usability Factor and Usability Criteria

The analyzing from the previous studies resulted several of finding which assist identifying what kind of usability elements that can be embedded in digital textbook interface before publish. The reviewed found that the dominant usability factor that has been used were efficiency, effective and satisfaction. In order to ensure the review of usability properties more holistic, the review also involved previous mobile application usability study and the suggestion from standard or the dominant scholar. From there, it was found over 40 types usability properties has been employed which the dominant usability properties encompass accuracy, consistency, navigability, readability, aesthetic, visibility and visual appearance. As previously mentioned, this review focuses on research into digital textbooks in a total of 17 papers. The organizing of usability factor and usability properties have been done through summarize table (Tables 5, 6). Table 4 presents the overview of the usability study for this reviewed paper. Based on all analyzing, this paper suggests that there is a lack of standardization of the usability elements that can be embedded when designing the digital textbook interface. In addition, there is also insufficient on conducting empirical research involved identifying the usability factor of digital textbook interface.

Table 5 Usability factor adopted in the previous research
Full size table
Table 6 Usability factor and usability properties
Full size table

Efficiency, Effective and Satisfaction as the Dominant Usability Factors Used Digital Textbook Research

Efficiency is the most dominant of usability factor that has been used in the previous research. It was found 6 out of 17 previous studies adopted efficiency as usability factor for digital textbook research [1, 26,27,28,29]. Followed by 4 studies used effective as usability factor in the digital textbook usability research [1, 26, 28, 29]. Satisfaction is the third most frequently used in the digital textbook usability research as usability factor. Four studies referred satisfaction as usability factor based on the reviewed paper.

From the finding, generally the usability factor is based on the suggestion from the dominant scholar such as [30,31,32,33]. All this scholar had suggested several of usability factor however, the dominant usability factor that has been adopted in previous research is efficiency and effective. While, worldwide standard also influenced the determination of usability factor in digital textbook research. This can be seen through adoption of satisfaction as usability factor. Several of worldwide standard employ satisfaction as usability factor such as SQuaRE-25010 [34], ISO/IEC 9126-1 [35] and ISO 9241-11 [36]. This finding also was supported by the comparison that has been done with the previous mobile application usability studies (Table 6).

The dominance of these three usability variables has shown that the usability factor is the most influential factor and when developing the digital textbook interface, the factors must be regarded and integrated. Based on the literature, efficiency refers to the software product’s ability to allow customers to spend adequate quantities of funds on the efficiency obtained in a specific use context. While effective is described as the software product’s ability to allow users to perform specific duties accurately and fully. Satisfaction referring to users ‘ subjective reactions to their emotions when using the software (i.e. the customer is satisfied or satisfied with the system) [37].

Learnability and Accessibility as Usability Factors Employed in Digital Textbook Research

A total of two empirical research involving digital textbook used learnability [38, 39] and accessibility [27, 40] as usability factor. Even though these two factors were used in two previous studies, but these factors look relevant for the digital textbook context. The reason is that the role of the digital textbook has altered in comparison with the printed textbook as proposed by Rezat [41] which can be explained in the student–textbook–mathematical information used for the triangular association of the textbook; the student–teacher–textbook; the teacher–textbook–mathematical knowledge (didactic); and the student–teacher–mathematical knowledge (represented by the textbook as a textbook). Digital textbook interface with multimedia components or interactive characteristics can be intended with the growth of digital technology. This latest development calls the educational scholar to concern the connection between student and digital textbook in term of engagement [42, 43]. Interactive element inside the digital textbook can increase engagement of the student [39]. Therefore, the element should be designed by paying attention to the student capability. Without concerning the student capability, it makes the student as a user become confused because of the product difficult to understand how it is work [44]. Under this circumstance, it will affect the learnability of digital textbook and impede the student intention to achieve their goal.

The justification was aligned with the teaching concept proposed by the dominant scholar, who referred to it as the ease with which to master the features required to attain particular goals. It is the capacity of the software product to allow customers to feel immediately productive use of the software product and then rapidly discover other new (for them) features [37].

Since the digital textbook purpose is to use of both classroom and different method of learning, Universal Method of Learning Principle could be possible to be adopted in the digital textbook design [45]. The principle was used to accommodate a variety of students needs and students’ capability level. Besides, the fundamental of UDL was developed for helping disable students (physically and mentally) in the special education to ensure them could be able to follow the learning based on the standard curriculum has been developed [46]. Therefore, accessibility as usability factor should be relevant to be employed in digital textbook design.

On this grounds, some kind of deficiency (e.g. visual, hearing, psychomotor) was included in the definition of accessibility [37]. According to Seffah et al. [37], accessibility is referred to as the capacity of a software product to be used by individuals with different demands. Furthermore, the definition aligned with the necessity of the UDL principle to satisfy distinctive learners ‘ demands.

Other Adopted Usability Factor

It was found that only one paper employed several types of usability factor [40] which obviously difference with other research such as operability, visibility, convenience, consistency, completeness and aesthetics. The research aimed at investigating the student’s conduct towards digital textbook based on usability factor. The study used as a research framework the Technology Acceptance Model (TAM). Usability factor was used as an indicator to determine the usage conduct perceived to be easy to use. However, the usability factor was adopted based on several of standard and from the suggestion of dominant scholar in usability study. For example, the visibility is one of the usability factor was suggested from Nielsen [31] and Norman [47]. While for the rest of usability factor was suggested from standard neither SQuaRE-25010 nor ISO/IEC 9126-1 or the model like Theoretical Model: E-Learning Courseware [48].

Generally, usability factor and usability properties were used interchangeable to form usability factor from the reviewed paper. Under this circumstance, there was not a clear boundary between these two elements which affect the process of digital textbook development.

Definition and Concept of Usability Properties

The concept and definition of the usability properties has been discussed among western scholar almost 30 years ago. Usability properties can be defined as the features or elements embedded in the interface that are designed in a product to enhance the capability and interaction between the product and user [30, 31, 49,50,51,52,53]. Usability properties also are the features of the product encompass simplicity, comfort, navigability, recognition, relevancy [48].

In this paper, to ensure the reviewed of usability properties more comprehensive, the study also compared and reviewed the previous usability study in mobile application and ebook usability studies. The reviewed paper is not only based on [54] paper but also involved the research of usability model for the last 20 years ago (Table 6). By doing that, grey situation could be avoided to differentiate between usability factor and usability properties in digital textbook research.

High and Low Dominant Usability Properties

Almost 20 years ago, there are 48 studies has been done to investigate the usability factor of product such as mobile application, ebook and digital textbook (Table 6). The extended to review of usability studies in mobile application and ebook because the limitation of usability properties discussed in digital textbook research. The constraint of usability properties discussed in previous research can be seen in Table 4. All the finding focusing on usability factor and few research used usability factor and usability properties interchangeable [55,56,57,58,59]. In principle, 15 dominant usability properties have been identified from analyzing of mobile application studies, ebook usability studies and digital textbook usability studies (Fig. 2).

Fig. 2
figure 2

Frequency and percentage of usability properties adopted in previous research

Full size image

Thus, these dominant usability properties should be concerned by the publisher of digital textbook since it gives a high impact to the quality of digital textbook. The dominant usability properties that has been identified encompass accuracy, aesthetic, appearance, completeness, comprehensibility, consistency, content, feedback, information, navigability, readability, seeking, simplicity, visibility and privacy. The specific definition of each usability properties as in Table 7.

Table 7 Definition of usability properties
Full size table

Discussion

In this research, the prior research literature to make digital textbook simple for learners to use was systematically evaluated in terms of components of usability. Analyzing the trend of usability factor and usability properties from the reviewed paper identified that three types of usability factor and 15 usability properties dominated adopted in digital textbook research.

The trends were produced by a strict analysis based on two databases which involved 17 papers on usability studies of the digital textbook. The findings show that the usage of usability factor and usability properties interchangeable. The scenario could lead to no separation between these two usability elements. It is found that 10 out of 16 studies adopted the usability factor which come from the worldwide standard namely ISO/IEC 92141-11 [36]. While the rest of the other study applied interchangeable the usability factor and usability properties.

Therefore, this study proposes that dominant usability factor and usability properties can be considered to be applied when designing digital textbook interface as it is look more stable and relevant. The usability factor and usability properties are the usability elements which interplay each other and enabler to make the user satisfy towards digital textbook. The user satisfying is very important since the feel could lead to the continue of intention to use the digital textbook [60]. Several of empirical studies also aligned with the claim. By adopting appropriate usability element when designing the interface of digital textbook could fulfill one of the digital textbook quality as suggested by worldwide recognized standard such as SQuaRE-25010 [34].

The usability elements should be classified as usability factor and usability properties in order to make a clear boundary. The classification aligned with the usability model of e-learning that has been introduced by western scholar [48]. The illustration as in Fig. 3 show the propose of usability elements that need to be embedded in the digital textbook interface during publishing process. The usability elements are classified in two layers of plane to form digital textbook interface which also referred as front end. The first plane layer is referred as usability properties. This layer will determine what kind of characteristics or features that need to be designed in the digital textbook interface. The second layer which referred as usability factor is an outcome of user feel or the end result of the digital textbook after used it. These two elements play a key role when the additional of features and variety of presentation technique embedded in digital textbook interface [61, 62]. This fact also supported by well-known western editor which claim that element design of interface will be very important for the screen display product (ebook) compared to the printed display product (printed book) [63].

Fig. 3
figure 3

Two layer of usability elements consists of usability factor and usability properties needs to be considered to form digital textbook interface

Full size image

Even though learnability and accessibility are the lowest dominant adopted as usability factor in the previous study but the elements relevant to be considered as it could give significant impact to the digital textbook purpose. The aim of the digital textbook can be defined in two perspectives namely student-centered learning environment and teachers centered learning environment [20]. Therefore, to meet the perspective the digital textbook has been published with multiple format such as PDF and ePUB which consist of audio, video, animation and simulation [64]. The elements will enhance the interaction between student and digital textbook during learning process whether inside class or outside class. Every learning process has their own objective that need to be mastered by student.

To accomplish the intention, learnability is important when designing the digital textbook interface to be embraced as a usability factor. By embracing the factor, digital textbook design will automatically determine which characteristics may be mastered by learners during the teaching phase to achieve specific goals [37]. Accessibility also relevant to be extended as usability factor in digital textbook context. As claimed by dominant educational psychology scholar last 20 years ago, providing numerous ways of accessing to the educational materials will uphold the principle of Universal Design [65]. As a result, the design of digital textbook interface should allow the student to get the information in a variety of procedure. Indirectly, the spirit of inclusiveness for supporting multiple needs of students and diverse learning style could be achieved [46, 65]. An empirical study also has been done before by adopting Universal Design for Learning (UDL) principle in the design of digital textbook. The research found that UDL principle was a very useful tool to communicate on how to design the features of digital textbook among editor, writer and programmer [45].

Future Scope

Almost 30 years ago, several of dominant scholar in the usability study was determine the conceptual and definition of usability [30,31,32,33, 66]. While, within that time, several of standard were established to standardized the definition and conceptual of the usability. For example, ISO/IEC 9126-1 [35], ISO 9241-11 [36] and SQuaRE-25010 [34]. However, the conceptual of usability was different each other where sometimes it refers as usability attributes or usability criteria or usability properties. This condition is getting critical in digital textbook usability studies where the term used interchangeable without a clear boundary.

Therefore, our literature review indicates that the usability aspects that can be used when developing the digital textbook interface are lacking in standardization. Furthermore, there is also a theoretical absence of sound to conduct usability studies. All of these weaknesses express three significant problems that lead to future study scope that we will discuss next.

First, the usability properties need to be categorized into several components based on usability factor. The categorization will assist the editor to conduct a quality control toward digital textbook layout and interface design systematically. The usability element lead to new principles need to be adopted for the publishing digital textbook specifically in terms of development and production of digital textbook. The principle needs to take seriously consideration since consumer preferences widely difference because of the availability of content can be in various format and display mode [67]. The empirical research needs to be done to identify the most relevant usability factor and usability properties that meet the student satisfaction.

Second, the new emergence of worldwide standard should be concerned since the concept and definition of usability is difference compared to the previous standard. For instance, ISO/IEC 9126-1:2001 [35] was substituted by ISO/IEC 25010:2011 (Quality Requirements and Evaluation of Software Products) [34]. Several usability factors have been rebranded as understandability is renamed “recognizability” and “attractiveness” is renamed “user interface aesthetics.” Analytical effect of conceptual usability therefore needs to be achieved to prevent misunderstanding of the present standard requirement.

Lastly, most digital textbook usability surveys were not theoretically underpinned. For this reason, in future studies it is necessary to consider more appropriate integration between two or more theories, such as integration between TAM and flow theory. Future research also requires to consider using the least partial square structural equation modeling technique for analyzing structural equation modeling if the study samples are less than 100 learners. The reason is that the trend from the past research showed only between 100 and 200 the amount of samples. Furthermore, the modelling technique for partially least square structural equation (SEM-PLS) is sufficient to be used under circumstances of tiny to medium sample size and non-distribution sample [68]. In addition, the study could define the magnitude of the connection between usability characteristics and happiness or behavioral intent to use digital textbook by using SEM.

Conclusion

This systematic review has emphasized that a clear boundary definition and concept between usability factor and usability properties should be established to ensure the design of digital textbook interface as best as possible. Besides, coordination between editor, writer and programmer can be done properly since all the parties understand what the usability factors and usability properties are need to adopted in digital textbook. The detailed out of usability factor and usability properties for digital textbook interface design was highlighted. Finally, on the basis of recognized problems from the reviewed research, several suggestions are suggested for future studies. First, to define usability factor and usability properties, an empirical research must be performed. Secondly, the conceptual effect on usability must be recognized by replacing ISO/IEC 9126-1 with ISO/IEC 25010:2011 (Quality and Evaluation Software Product Requirements). The third is the theoretical sound that needs to underpin the digital textbook survey involving usability.