TOWARDS A COMPREHENSIVE MODEL OF HUMAN RESOURCE DEVELOPMENT RESEARCH AND PRACTICE: A SYSTEMATIC LITERATURE REVIEW

Although several models have been proposed to guide practitioners and academics in human resource development (HRD), there is sufficient manifestation that a new HRD model is required to address emerging HRD problems such as cultural variation and value systems across the globe, the need to strengthen the theoretical foundation of HRD research and practice, and the incompleteness of HRD research methods. The purpose of this study was to propose a comprehensive HRD model that will address these problems in order to make HRD more responsive to the current needs in HRD research and practice. Bozer and Jones’s (2018) seven-phase systematic literature review approach was adopted to gather data for this study. The available data were analyzed using thematic analysis. It was found that the proposed HRD model was capable of helping HRD practitioners to choose appropriate HRD practices that will match their culture and value systems, and guide them on the methods and theories to apply in their respective locations. On the basis of these findings, it was concluded that the proposed HRD model can address the problems currently facing HRD research and practice.


INTRODUCTION
Several models have been proposed to guide the study and practice of human resource development (HRD).The most dominant of these include human resource wheel (McLagan, 1989), parallel pathways of HRD and human resource management (HRM) (McGoldrick & Stewart, 1996), the three-legged stool (Swanson, 1999), HRD cube (Lynham, 2008) and HRD lattice (Mankin, 2001).These models have significantly contributed to the development of HRD in areas such as theory development (Lynham, 2008;Swanson, 1999), HRD definitions (McLagan, 1989), HRD components or functions (McLagan, 1989), distinction between HRD and HRM (Mankin, 2001;McGoldrick & Stewart, 1996), modes of HRD inquiry (Lynham, 2008), and performance domains of HRD (Lynham, 2008;Swanson, 1999).Notwithstanding the immense contribution of the existing models of HRD, the characteristics of today's knowledge era, such as knowledge being the basic production factor, Although there are no recent comments that signal the need for a new HRD model, the fact that no new model has been proposed after the preceding comments, suggests that one of the most pressing needs of HRD today is the proposition of a model that will address the emerging HRD problems such as cultural variation and value systems across the globe, the need for strengthening the theoretical foundation of HRD research and practice, and the incompleteness of HRD research methods and components.The current paper fills this gap by expanding on four major areas of HRD research and practice: (a) HRD theory, (b) components of HRD, (c) sources of variance in HRD research and practice, and (d) methods of HRD research.These areas are incorporated in the comprehensive HRD model which promises to tackle the current HRD problem.The proposed comprehensive HRD model will also inform and support HRD professionals and practitioners in resolving the learning problems of adult employees in their respective organizations.The model will also improve the professionalism of HRD by broadening its theoretical foundation to include adult learning theory.Finally, the inclusion of sources of variance in the study and practice of HRD will improve the fit between HRD practice and the prevailing socio-cultural, legal, and technological forces in the environment in which HRD practitioners operate.The rest of the paper is laid out as follows: the second part presents the methods for conducting this study whereas the third section discusses the findings of the study.The fourth section deals with the development of the proposed model while the fifth section outlines the implications, limitations and suggestions for further research.

METHODS
The author used the systematic literature review approach to gather data for this study.Torraco (2005) defined a systematic literature review as one that reviews the most relevant literature, critiques and synthesizes the available literature on a topic of interest in the most representative manner.According to Liberati et al. (2009, as cited in Guillaume, 2019), the systematic literature review method informs practice with scientific evidence by increasing transparency at every stage of the review process which allows the researcher to rely on explicit, systematic guidelines for reducing bias in the selection and inclusion of studies, appraising the quality of the included studies, and summarizing them objectively.Thus, by adopting the systematic literature review method, the researcher was able to review and synthesize existing knowledge about specific questions of practical relevance to HRD professionals (Polanin et al., 2017;Xiao & Watson, 2019).For its comprehensiveness, Bozer and Jones's (2018) seven-step systematic literature review approach was adopted in this study.The seven dimensions are: research questions, literature search, data set, inclusion and exclusion criteria, description of variables and coding accuracy, assessment of methodological accuracy and synthesis of information from eligible articles and books.The seven stages, and their application in this study are discussed in the following section.
The first phase of the systematic literature review relates to the formulation of research questions.These questions provide the foundation for key decisions about the types of studies to include, where those studies can be located, how to assess their quality and how to merge their findings and conclusions to form the themes into which they fall (Bozer & Jones, 2018;Xiao & Watson, 2019).In this study, three questions were formulated to guide the researcher: 1. How relevant is the adult learning theory to the study and practice of HRD? 2. What are the major sources of variance in HRD research and practice?3. How will a model that incorporates the sources of variance in HRD research and practice resolve current HRD problems such as cultural variation and value systems across the globe, the need to strengthen the theoretical foundation of HRD research and practice, and the incompleteness of HRD research methods?
The second phase of the systematic literature review is a literature search.In this study, all the articles were searched from credible sources, using search engines such as Google Scholar, Science Direct, PRO QUEST, EBSCO and ERIC documents.The researcher started with Google Scholar using broad keywords such as HRD, HRM, HRD theory, and HRD models.After reviewing and accumulating potentially relevant articles, the researcher refined the keywords and continued with the review process.This procedure was followed for the remaining search engines.As many as two thousand, four hundred and eighty (2,480) journal articles and books were obtained As it is a requirement of some higher impact journals that the Preferred Reporting Items for Systematic Review and Meta-Analysis (PRISMA) is incorporated into the review, the flow-diagram outlined by Moher et al. (2009) was adapted to extract the data of interest from each selected article or book as in Figure 1.There are four phases in the PRISMA: Identification, Screening, Eligibility, and Inclusion.The four phases are discussed in the next section.The PRISMA approach has the effect of minimizing bias in the review process.
Step diagram.Thus, the total number of resources gathered was three thousand and eighty-one (3, 081).
Step 2: Screening The initial screening of the three thousand and eighty-one (3, 081) journal articles and books gathered was done.This reduced the total number of resources to three thousand and fifty (3, 050).Thus, about two thousand, five hundred and fifty (2,550) searched materials were removed.The author considered materials published by reputable publishers such as Sage, Emerald, Elsevier, Taylor & Francis, and Routledge.
Step 3: Eligibility To form part of the review, articles and books must be published: (a) in English language peer-reviewed publishing houses; and (b) between May 1964 and January 2023.Articles and books that failed these criteria were excluded.About five hundred (500) records were assessed for eligibility.The available resources were assessed based on the research questions for this study, reducing their number to seventy.
Step 4: Inclusion and exclusion Four hundred records were published before May 1964 and thirty records were not published in English.A total of four hundred and thirty records were, therefore, removed.Thus, only 14% (n = 70) of the available articles and books met the criteria in this study.This suggests that the inclusion/exclusion criterion was very rigorous.
In the fifth stage, the researcher examined and described the state of the included studies as each pertains to a specific research question or concept in the study.This description aimed to provide an account of the state of the literature at the time of the study (Bozer & Jones, 2018;Guillaume, 2019;Xiao & Watson, 2019).Of the total works reviewed, 24 related to HRD models, 10 to theory-building, 6 to the dissimilarity between HRD and HRM, 12 to HRD components, 11 to modes of HRD inquiry, and 3 to HRD practitioner and the environment.Thus, 70 works were included.To be brief and concise, all eligible materials were coded by the researcher to align with the chosen methodology.
To minimize methodological errors, a critical appraisal of all eligible articles and books was done.Although the researcher believed that journal articles and books published by reputable publishers were usually of high-quality, and were included in the study, the researcher undertook a rigorous assessment of all included studies, based on description, selection, measurement, analytic, and interpretation biases (Xiao & Watson, 2019).After the assessment, two thousand, five hundred and fifty (2,550) searched materials were removed, indicating that the phrase; 'published in reputable peer-reviewed journal' cannot be relied on as sole criterion for assessing the quality of a study.
The final stage in the systematic literature review process is synthesis of eligible materials.The deductive thematic analysis approach was used.In the deductive thematic analysis, the researcher identifies the main themes around which the presentation of results is done, and the data obtained is analysed in relation to existing themes and frames derived from related literature review (Braun &  ).The analysis follows the theoretical interest of the researcher, making it an explicitly researcher-driven analytical tool (Braun & Clarke, 2006).The thematic analysis is still the most useful in capturing the complexities of meaning within a textual data set (Nowell et al., 2017).It is also the most frequently used method of analysis in qualitative research (Saraswati, 2020).

RESULTS AND DISCUSSION
The results were organized into five themes as follows.
Theory Building in HRD Many scholars have argued that there is no universally accepted theory underlying HRD research and practice (Fenwick, 2005;Harrison & Kessel, 2004;Lincoln & Lynham, 2007).For instance, according to Fenwick (2005), "HRD practice is still notgrounded in research-based theories; it is based on guess-work and outmoded thinking" (p.225).Swanson and Holton (2001) also argued that the existing theories of HRD are limited because HRD requires a sound theoretical foundation that will advance knowledge, guide research, and support practice.These arguments imply that there is still a considerable level of controversy over HRD theory in the literature.This controversy is evident from the calls by Torraco (2004) and Storberg-Walker (2006) to HRD professionals to propose a multi-paradigm theory of HRD research and practice.However, though there appears to be some veracity in the argument that HRD does not have a strong theoretical background, most studies in the literature have used and continue to use the three theories proposed by Swanson and Holton (2001), namely: the economic, psychological, and systems theories (Haslinda, 2009).As noted by Lynham et al. (2007) "the minimum theoretical foundations of HRD will be taken to include economics, psychology, and the systems theories" (p.152).
The debate on whether HRD has a strong theory or otherwise is necessary because ambiguity is necessary for preventing an academic area of study from being stagnant.According to Mankin (2001), "HRM has already gone through a similar experience, as the many debates of the last decade bear witness" (p.65).Nevertheless, having a strong theory is inevitable.Theories explain what a phenomenon is and how it works (Torraco, 1997).They advance knowledge, guide research and enlighten the profession in which they are applied (Keith, 2005;Van, 1989).Consequently, having a well-cultivated theoretical model allows professionals and practitioners of a discipline to place their work within appropriate perspectives (Bhattacherjee, 2012).

HRD Practice Models:
Five models emerged from the review of literature in this study: (a) the three-legged stool, (b) HRD cube, (c) HR wheel, (d) parallel pathways of HRM and HRD, and (e) HRD lattice.A review of these models is presented in the following.
The Three-Legged Stool: Swanson proposed the three-legged stool in 1999.
According to Swanson (1999), the stool is only an icon of the foundational theories of HRD, namely economic, systems, and psychological theories.Each of these theories, according to Swanson (1999) is unique, complementary and robust, and their CC-BY License GJDS, Vol. 20, No. 2, October, 2023 integration, more than any others, has been central in securing HRD as an area of study and practice.The stool has a platform which denotes the incorporation of the three HRD theories, which together conceptualize HRD into two main dimensions: (a) the theoretical framework of HRD, including psychology, economic, and systems theories, and (b) the performance spheres of individuals, processes, and organizations.The three-legged stool is placed on a rug which filters and or maintains the integrity of HRD and the organization (Stewart & Sambrook, 2012).
The three-legged stool is shown in Figure 2. The economic theory is built on survival and the primary drive of organizations, which holds that for its existence, the organization must engage in activities that generate more income than it spends.The theory captures the core issue of business efficacy in the utilisation of human resources.The theory provides two important guides to HRD practitioners.First, there is no sufficient number of productive resources held by anyone, and this poses several constraints on both the practitioner and the organization in terms of the choices and opportunities open to them.Second, HR professionals must incur expenditure only if it is expected to benefit the organization (Stewart & Sambrook, 2012).
The psychological theory derives from the behaviours and mental processes of people in the organization.Psychological theorists view HRD as a process involving the alteration of human behaviour through education and training (Lee, 2001).The third theory proposed by Swanson (1991) is the systems theory.The systems theory focuses on the complex interactions among the forces in the environment (Swanson  & Holton, 2001).In line with the systems theory, HRD may be considered as a subsystem of the whole organizational system, and, like all other sub-systems, can affect the functioning of the larger organizational system, which in turn, can affect HRD research and practice.
The HRD Cube: The HRD cube was proposed by Lynham in 2008.The model conceptualizes HRD as having three major components -theory, research and practice -which together explain how knowledge is created within and across the study and practice of HRD (Hurt et al., 2014;Lynham, 2008)  The X-axis includes theories about people, processes and performance (Lynham, 2008).In addition to Swanson's (1999) economic, psychological and systems theories, Lynham (2008) identified sociology, adult education, and political science.The Z-axis deals with the potential metaphysical views of HRD research and inquiry.Notable among these are positivism, post-positivism, and social constructivism (Lynham, 2008).According to Lynham (2008), in between these perspectives, there could be other sub-perspectives, such as ontology, epistemology, axiology, and theology.The Y-axis defines the outcome and performance of individuals, groups, organizations, families and communities at the regional, national, and global levels.and Stewart (1996), because of the ambiguity in the roles and responsibilities of the HRM function, HRD now supplements HRM rather than supports it.In this regard, a parallel pathway, otherwise known as parallel nexus now exists between HRM and HRD, where HRM performs four core functions: motivation, rewarding, selection and recruitment.HRD on the other hand provides the requisite training and education for the selected and well-motivated employees.The model also postulates that HRM and HRD have their own sets of plans and policies, as well as functionality support in the corporate strategy.Hence, the distinction is based on the conceptual differences between strategy and practice, and between function and process.The model is presented in Figure 4.The HRD Lattice: Mankin (2001) proposed the HRD lattice after reviewing the dominant HRD practice models in the literature.The HRD Lattice is presented in Figure 6.

Distinction Between HRD and HRM
In most countries, there is hardly any clear-cut distinction between HRD and HRM (Konan, 2010;McLean & McLean, 2001).In fact, HRD is perceived as one of the many components of HRM, which is required to support the HRM function in terms of employee training and development.Several HRD scholars have attested to this, arguing that the HRD function was traditionally subsumed within the HRM department where the individuals performing those functions often had very little or no background training in HRD (Haslinda, 2009;Rowold, 2008).For instance, according to Haslinda (2009), HRM is the only label that represents human resource management, beginning from recruitment and selection to training and development, through reward management to retirement from the firm.Blake (1995) also argued that there is no evidence that the ASTD which approved the term "HRD" intended to make it a separate field of study.
In stark contrast to the preceding arguments however, other studies (Jacobs, 1990;Lynham, 2008;Mankin, 2001;McGoldrick & Stewart, 1996) (2006), in several countries (North America, Western Europe, and Asia) today, HRD is accepted as an academic discipline.According to the authors, in the listed countries, there are a multitude of degree programmes in HRD, coupled with the growth of the university forum, the proliferation of HRD journals, the academy of HRD, and a range of international research conferences for HRD professionals and practitioners.In McLagan's (1989) redesigned HR wheel, both HRD and HRM were treated as separate fields of study and practice.McGoldrick and Stewart (1996) have also drawn a distinction between HRD and HRM as separate fields of study and practice.
Similarly, according to Mankin (2001), although the two subjects are integrated and similar to some extent, practitioners and those in academia must both understand that each subject has its distinctive characteristics.(Ellinger & Bostrom, 2002;Greenhaus, 1987).Career development, on the other hand, consists of the development of employee skills, interests, and attitudes; the appraisal of the work environment; and the alignment of these attributes in decision-making relating to the determination and development of employee careers.The third component, organizational development, refers to the planned interventions usually employed by management to enhance the functioning of organizational members and the organization at large (Stewart & Sambrook, 2012).Although the three components of HRD have been used severally in the literature, Maughan (2005) suggests that training through the electronic performance support system could be used to improve trainee performance by providing a just-in-time learning experience and integrating the learning experience with the operational experience.

Modes of HRD Inquiry
This section reviews the three main scientific research approaches that may be deployed in HRD research.They are: (a) epistemology, (b) ontology, and (c) methodology.Epistemology focuses on the nature, forms and how human knowledge may be acquired or communicated to other people (Cooksey & McDonald, 2011).According to Hurt (2010), there are four forms of epistemologiespositivism; post-positivism; interpretive; and critical science.A review of the literature has shown that the positivistic and post-positivistic epistemologies continue to dominate much of HRD research due to the neglect of the interpretive epistemology (Cooksey & McDonald, 2011).However, because HRD is a social science discipline, it can best be viewed as a subjective rather than an objective field of study.It can, therefore, not be best understood by using purely scientific approaches (Cooksey & McDonald, 2011).
Ontology is the study of reality (Schwandt, 1997).According to Delbridge (2006), ontology is fundamental rather than optional in social science research.Unfortunately, to-date, ontological assumptions have been disappointingly absent in CC-BY License GJDS, Vol. 20, No. 2, October, 2023 research about employment or human resource-related investigations, which, according to Harney (2014) have delayed progress and prevented human resourcerelated disciplines from accommodating current changes in the employment relationship.The third approach is methodology.According to Rehman and Khalid (2016), methodologies guide the choice of data and data collection tools used by researchers.

Analysis of data from the Literature Review
This section focuses on the issues that emerged from review of literature.In all, these may be grouped into four: (a) the scope and content of HRD practice models, (b) the HRD practitioner and learning theories, (c) the strategic role of the HRD practitioner, and (d) the HRD practitioner and the business environment.The analysis and interpretation of these issues are fundamental to the comprehensive HRD model.

Scope and Content of HRD Practice Models
The review of HRD practice models has revealed that HRD research revolves around five major themes, including theory building, components of HRD, the distinction between HRD and HRM, modes of HRD inquiry, and performance domains of HRD.Theory building occupied a major part of Swanson's (1999) three-legged stool of HRD.
In the HRD cube, Lynham (2008) also identified six theories, including economics, systems, psychology, sociology, anthropology, political science and adult education.Theory building was, however, not mentioned in the parallel pathways model, the HR wheel, and the HRD lattice.In terms of HRD components, whereas McLagan (1989)  In the literature today, the components proposed by McLagan (1989) have frequently been used (Lynham, 2008).
The distinction between HRD and HRM was clearly manifested in McGoldrick and Stewart's (1996) parallel pathways model.The HR Wheel by McLagan (1989) and HRD lattice by Mankin (2001) also made similar propositions about the distinction between HRD and HRM.Finally, in terms of the modes of HRD inquiry, only Lynham's (2008) HRD cube model identified social constructivism, post-positivism and positivism.The cube model also identified eight performance domains of HRD, including group, organization, community, family, process, regional, national, and global.The review of HRD practice models was important because it allows one to identify the major areas of concentration for HRD modelling.

The HRD Practitioner and Learning Theories
The extant literature has shown that the HRD practitioner performs three functions: (a) training and developing employees, (b) organizational development, and (c) career development (Ellinger & Bostrom, 2002;Greenhaus, 1987).Technically, CC-BY License GJDS, Vol. 20, No. 2, October, 2023 though these functions are designed by the HRD practitioner, they are often implemented by employees who require some amount of learning that will enrich their skills, knowledge and experiences for a successful implementation of HRD plans.Thus, if employees have to make things happen, then the HRD practitioner has the responsibility of facilitating employee and organizational learning.According to Ellinger and Bostrom (2002), to facilitate teaching and learning, the facilitator must understand theories and principles of learning, and how they enhance a learner's training and education.In light of this, the HRD practitioner today must understand how employees learn at work (Greenhaus, 1987), and since employees are generally adults, the most relevant type of learning at the workplace is adult learning.Knowles (1968) described adult learning as the learning that allows adults to gain knowledge and expertise which are unique to adults.The approach is based on the doctrine that adult employees bring significant insights, experiences, and connections from prior learning to new learning situations.According to Knowles (1968), it is important to rely on these unique backgrounds to understand how adults learn.

The Strategic Role of HRD Practitioners
The literature review has shown that HRD practitioners have taken on a strategic role in addition to their traditional role of implementing strategies from top-level management.This new role places the HRD practitioner in the main-stream strategic management, where he contributes to the strategic directions and success of the organization (Lee, 2010).The strategic role of the HRD practitioner implies that the way we theorize and practice HRD has shifted from internal functioning under a stable bureaucratic environment to a constantly changing environment where coercive pressures from forces such as economic, legal, technology and culture occur (Garavan & McGuire, 2010).This calls for the alignment of the HRD function with the strategic goal of the firm which requires the analysis of forces in the external environment.

HRD Practice and the Business Environment
The literature review has also revealed that the forces in the business environment, such as government regulation, technological advancement, socio-economic transition, and a country's cultural/value system have significant influence on the study and practice of HRD (Garavan, 2007;Jacobs, 2011).This recognition is quite contemporary.Traditionally, HRD was primarily an internal organizational function that aimed at sustaining human capital to meet the production goals and objectives of the organization (Jacobs, 2011).The practitioner was confined within the organization and had little to do with issues relating to the forces in the external environment since these functions were reserved solely for line managers (Garavan & McGuire, 2010).Today, and as said earlier, in addition to the traditional functions, HR now plays a key role in formulating strategies for the organization as a whole.For instance, as noted by Lee (2010), "HRD is no longer a bureaucratic sub-function.It permeates the organization and incorporates subjects such as law, philosophy, politics, and policy (p.16).Thus, HRD practice is now grounded in the external environment, where HRD practitioners have the added responsibility of scanning the general environment for people-related issues.

Identified gaps and Development of the Proposed Model
Two questions emerged from the literature review: 1) How adequate are the dominant HRD models in addressing the current HRD problem? 2) How will the proposed comprehensive HRD model address the current HRD problem?These questions are addressed in the following.
How adequate are the dominant HRD models in addressing the current HRD problem?
While the dominant models have provided a strong basis for the study and practice of HRD in the past, recent changes in the organizational structure and operations, as well as changes in external environmental forces such as socio-economic influence, government regulation, technological advancement, and a country's cultural/value system have created the need for a model that can address HRD problems relating to cultural variation and value systems, the incorporation of technological change in the HRD system, and the inclusion of other environmental forces such as legal, social and economic (Fenwick, 2005;Haslinda, 2009).Again, looking at its strategic role, HRD practitioners can no longer focus solely on internal organizational activities such as, employee development, job satisfaction, and performance management, as they must also deal with the forces in the external environment.

How will the proposed comprehensive HRD model address the current HRD problem?
Much like the five dominant HRD models, the proposed comprehensive HRD model supports the existing theories that guide HRD research and practice (e.g.systems theory, economic theory, and psychological theory); the major functions of HRD (e.g.employee training and development, organizational development, and employee career development); the basis of HRD research (e.g.positivist philosophy, interpretive philosophy, and the mixed methods); and the levels of HRD activity (e.g.individual, organizational, and national).However, unlike the existing models, the proposed HRD model has incorporated an adult learning theory which deals with how adults learn.The inclusion of this theory is informed by the assumption that employees are generally adults who are self-directed and expect to take responsibility for decision-making (Knowles, 1968).The inclusion of the learning theory also implies that HRD is grounded in learning and research which are fundamental in performance management.The author of this study is aware that Lynham (2008) included adult education in the HRD cube as one of the major theories of HRD.However, adult education or andragogy is just one of the adult learning theories.Other adult learning theories include transformative learning, selfdirected learning, project-based learning, and experiential learning (Knowles, 1968).
The comprehensive HRD model also incorporates the electronic performance support system (EPSS).The EPSS is an integrated system that is easily accessible to all employees in the organization.The system is designed in such a way that it provides self-tuition to employees with little or no support from practitioners and supervisors (Maughan, 2005).By implementing the EPSS, the degree of supervision by HRD practitioners is reduced.The comprehensive HRD model has also incorporated the four main external environmental forces that usually influence HRD CC-BY License GJDS, Vol. 20, No. 2, October, 2023 research and practice -socio-economic forces, government regulation, technological advancement, and the country's cultural/value system.In this study, these forces are known as the sources of variance in HRD research and practice.By incorporating these forces, the HRD practitioner is now added to the main-stream strategic management, whose decision making must go beyond internal operations.Finally, the proposed model encourages HRD practitioners to also consider the ontological and epistemological principles underlying HRD research in general.McLagan (1989) in the HR wheel.The three components have been discussed in the literature review section of this study.In addition to the three known components of HRD, this study proposes performance support systems, especially the electronic performance support system (EPSS) as one of the major components of HRD.The argument for the inclusion of performance support in HRD components is that the ultimate goal of training and development, organizational development, and career development is performance improvement (Gery, 1989;Maughan, 2005;Swanson & Holton, 2009;Wheelen & Hunger, 2012).The inclusion of performance support, therefore, serves as a signal to HRD professionals that as they are training and developing their employees, and developing the organization and employee careers, they must anticipate the effect of these functions on the performance of individual employees and the organization at large.This mindset has the advantage of tunning HRD research and practice, as well as the work of HRD professionals towards achieving superior performance (Wheelen & Hunger, 2012).

The
The electronic performance support system is preferable because it provides selftuition to employees with little or no support from practitioners and supervisors (Maughan, 2005).Traditional on-the-job training has evolved to the extent that some instances of training are built into the work process itself.As a result, by adopting the electronic performance support system, much of the traditional training of employees will be eliminated since employees can use the new system to learn while working on the job.According to Swanson and Holton (2009, p. 439), the "EPSS contains four main components: (1) tools (to perform a job), ( 2) information (needed to do the job correctly), ( 3) advice (for the difficult parts of the job), and ( 4) training (to extend the employees' knowledge and skills)".The preference for the electronic performance support system does not imply that other non-electronic support systems are precluded in HRD.The component being added here is a performance support system, of which the electronic performance support system is a part.
Part 2: Sources of Variance in HRD Research and Practice Following the literature search, it was found that four uncontrollable forces in the external business environment (socio-economic, technological advancement, government regulation, and a country's cultural/social system) have a substantial CC-BY License GJDS, Vol. 20, No. 2, October, 2023 impact on HRD research and practice (McLean, 2004).The four factors usually dictate the kind of skills and abilities needed by a country.The same factors even dictate the type of organizations and the choice of employee careers in a country (Fenwick, 2005).The underlying assumption is that the economic, socio-cultural, and technological environments of developed countries differ from those of developing and semi-developed countries.Furthermore, the nature and scope of training and developing employees, organizational development, and employee career development for these countries also differ.Thus, the essence of incorporating these forces in the model is to caution HRD practitioners that although the proposed model may be universal, its application may vary from one country to another due to the nature of the forces prevailing in the practitioner's respective business environment.The HRD practitioner is no longer required to follow rigid HRD practices which sometimes deviate from the environment where they operate.

Part 3: HRD Research and Theory
The proposed model recognizes the fact that research in HRD must incorporate all the principles of scientific research as applicable to other disciplines.The import of this inclusion is to remind HRD professionals that good HRD research must also consider the ontological and epistemological principles underlying the study.Epistemology helps to extend, broaden and deepen one's understanding of her field of study (Richards, 2003;Scotland, 2012) while ontology allows her to conceptualize the nature and effect of reality; positions her understanding of the study problem, its importance and how to properly contribute to its resolution (Delbridge, 2006;Richards, 2003).
The comprehensive HRD model is also built on four main theories -the economic, systems, psychological, and adult learning theories.The economic, systems, and psychological theories were first identified by Swanson (1999) in the three-legged stool and supported by Lynham (2008) in his HRD cube.The three theories have been discussed in this study.The present study, however, proposes the addition of the adult learning theory as a major theory in HRD research and practice.The inclusion of adult learning theory is based on the assumption that employees require some learning before they can successfully implement HRD interventions from their superiors.Although scholars such as Kumar et al. (2015), Swanson and Holton (2009), Watkins (1989) and Knowles (1968) have recommended adult learning theory in HRD research and practice, its inclusion in the current model buttresses this stand, signifying the need for HRD practitioners to know how adults learn and successfully perform their assigned roles and duties (Watkins, 1989).The proposed comprehensive HRD model, using a systematic literature review attempts to build a formidable and all-inclusive HRD model that meets some professional standards in our time.It is for this reason that some relevant findings have been incorporated into the model.

Part 4: Levels of HRD Activity
The fourth core aspect of the proposed comprehensive HRD model relates to the major levels of HRD activity or the domains of HRD performance.These include the individual, organization, national, and global.The four levels were incorporated into CC-BY License GJDS, Vol. 20, No. 2, October, 2023 the HRD cube by Lynham (2008).The essence of incorporating the levels of HRD activity in the present model is to prompt practitioners that differing interventions are required at each level of HRD activity.For instance, HRD at the individual level focuses mainly on the "human aspect" of HRD, including self-efficiency and motivation to learn (Garavan, 2007).At the organizational level, practitioners focus mainly on the "resource aspect" of HRD, adopting interventions, such as the maximization of resources and achievement of full employee potential and employability (Lynham, 2008).HRD at the national level focuses on the provision of the requisite education and human capital for all nationals (McLagan, 1989), while HRD at the global level deals with how HRD professionals address the geographic, linguistic and cultural differences among their workforce when performing their functions.

Part 5: Areas Common to HRD and HRM
The comprehensive HRD model is placed on top of a "base" that denotes the common areas shared by both HRD and HRM.The significance of the base is that HRM and HRD share common functions such as, job and organizational design, HR planning, and staffing.However, they also have slightly different objectives.Thus, while HRD is mainly concerned with the employee's personal and professional growth and development to suit organizational goals, HRM primarily focuses on the optimum use of human resources by nurturing better working conditions for the workforce and performing other functions such as benefit and compensation management, labour relations, diversity, and compliance management.To show where both subjects part ways, an arrow called the "Veil-Point" is placed beneath the base.The veil point is the point of separation between HRD and HRM.In sum, the current study recognizes the fact that though both HRD and HRM deal with the same human resources, they can be treated as separate areas of study since each discipline has a differing objective whose achievement requires separate HR functions.
The proposed model is provided in Figure 7.

Implications for HRD Practice
The current study has several practical implications.Generally, the study found that the proposed HRD model was capable of helping HRD practitioners to choose appropriate HRD practices that will match their cultural and value systems.The model is also expected to guide HRD practitioners on the methods and theories to apply in their respective locations.Second, the study revealed that although HRD practitioners are those who design HRD programmes, employees are the people who implement those programmes.Consequently, HRD practitioners and their organizations must assist employees to learn and acquire the requisite skills and knowledge through adult learning.Finally, the inclusion of performance support serves as a signal to HRD professionals that as they train and develop their employees, they must anticipate the effect of these functions on individual and organizational performance.This mindset has the advantage of tuning HRD research and practice, as well as the work of HRD professionals towards achieving superior performance.

CONCLUSION
The current study aimed to propose an HRD model that will address current HRD problems as identified in the literature review.To achieve this goal, the author conducted a systematic literature review which allowed him to obtain a huge and more focused body of literature from which he proposed the comprehensive HRD model that incorporated the core issues of HRD research and practice for tackling the current HRD problem.First, the inclusion of adult learning theory in HRD research and practice demonstrates the need for HRD professionals to ensure that their employees are given the requisite skills and knowledge through continuous learning so they can understand and successfully implement HRD interventions from their superiors.Second, the inclusion of the four major external environmental forces such as economic, socio-cultural, technological and a country's cultural/social system in the comprehensive HRD model is a signal to HRD practitioners that although the proposed model can be applied everywhere, its application may vary from one country to another due to the nature of the forces prevailing in the practitioner's respective business environment.The nature and type of these forces are the source of variance in HRD research and practice which usually dictate the kind of employee skills and abilities needed, the type of organizational ownership and the choice of employee careers in a particular country.Third, the implementation of the electronic performance support system as incorporated in the comprehensive HRD model provides a self-tuition mechanism for employees.It also reduces the degree of supervision by HRD practitioners.Fourth, the recognition that ontological and epistemological principles also underly HRD research and practice helps to broaden and deepen one's understanding and conceptualization of HRD as a field of study.Finally, the combination of adult learning theory, sources of variance in HRD research and practice, and electronic performance support system is expected to resolve current HRD problems such as cultural variation and value systems across the globe, the need to strengthen the theoretical foundation of HRD research and practice, and the incompleteness of HRD research methods.

Figure 1 :
Figure 1: Flow of information during the review Source: Adapted from Moher et al., 2009.
1: IdentificationThrough several backward and forward searches, the researcher identified two thousand, four hundred and eighty (2,480) journal articles and books through database search, and another six hundred and one (601) through manual search.The 601 other articles are recorded in the box on the top right-hand side of the flow

Figure 2 :
Figure 2: The theoretical foundations of human resource development Source: Adopted from Swanson (1999).
. The three components, their synergic relationships, and intersectionality are presented in the form of interacting axes which portray HRD as a multi-dimensional area of study.The interacting axes are: (a) informing theoretical foundations (X-axis), (b) domains of outcome and performance (Y-axis), and (c) modes of knowledge and inquiry (Z-axis).The cube is presented in Figure3

Figure 4 :
Figure 4: Parallel pathways of HRM and HRD (McGoldrick & Stewart, 1996) Source: Adopted from McGoldrick and Stewart, 1996.The HR Wheel: The HR wheel was proposed by McLagan in a study sponsored by the American Society for Training and Development (ASTD) in 1989.The Wheel is presented in Figure 5.

Figure 6 :
Figure 6: The HRD Lattice Source: The HRD Lattice, Mankin (2001) The first version of the model, the HRD continuum, comprised two dimensions: (a) the reactive, and (b) the proactive.Mankin (2001) revised the original model and proposed the HRD lattice.The new model proposed four major roles of the HRD practitioner: (a) communication and information, (b) actions and behaviours, (c) learning and knowledge and (d) management and leadership styles.According toMankin (2001), any significant change in organizational strategies, structures, cultures and patterns of interaction among the four dimensions in the HRD lattice has the potential of changing the composition of the model.Mankin (2001) therefore, cautioned both practitioners and scholars to hold HRD as an ambiguous and ever-growing academic discipline since it is this ambiguity that differentiates it from all other fields of study.
identified employee training and development, organizational development, and career development, McGoldrick and Stewart (1996) identified learning, education, and training.In Lynham's (2008) HRD cube, four components were identified: (a) communication and information, (b) actions and behaviours, (c) learning and knowledge and (d) management and leadership styles.

Five
Parts in the Proposed Comprehensive HRD Model Part 1: Components of HRD Practice In the comprehensive HRD model, HRD is defined to include training and development, organizational development, career development, and performance support.The first three were originally proposed by The parallel pathways model was proposed by McGoldrick and Stewart in 1996.The underlying assumption of the model is that HRM and HRD are distinct areas of research and practice.According to McGoldrick GJDS, Vol. 20, No. 2, October, 2023The Parallel Pathways of HRD and HRM: Before McLagan's (1989) study, only training and development were identified as the components of HRD.McLagan (1989) identified two additional components: (1) organization development and (2) career development.The training and development component focuses on changing, shaping, and improving the skills, knowledge and abilities of individual employees