Framework to Support the Transfer of Innovative Interventions in the Disability Field: Lessons from the Transferability of Complex Interventions in Public Health: A Review

Abstract

Innovative initiatives emerge in line with the recommendations of the United Nations Convention on the Rights of Persons with Disabilities. They are often place-based, context-dependent, and are not easily adapted for use in other contexts. It raises the question of their transferability. This concept has been studied in the field of public health. To explore the conditions surrounding the transfer of disability interventions, this study aims to determine the advances related to the transferability of complex interventions in public health. A review was conducted. Data were analyzed according to the concepts and terms used to describe the terminology related to transferability and the processes used to manage, assess, and report transferability. Fourteen papers fulfilled the inclusion criteria. The analysis shows that different terms and concepts are used. Numerous tools or frameworks have been developed to structure the identification of transferability factors or adaptations and usually require the involvement of stakeholders. Considering context is central. Finally, we identified a lack of reporting. This review provides a structured and operational framework for various concepts, including transferability as a form of knowledge generation, and implementation/adaptation as proactive actions. It emphasizes that a holistic approach to assessing transferability involves shifting the focus from transferability factors to understanding mechanisms of change and their interactions with the context. The review highlights the pivotal role of stakeholders in generating knowledge, capturing diverse contexts, and prioritizing information. Ultimately, this work will serve as a valuable foundation for guiding methodological developments on transferability in the field of disability.

1. Introduction

The United Nations Convention on the Rights of Persons with Disabilities (UNCRPD) has brought about significant changes in the field of disability rights, emphasizing the importance of changing attitudes and practices at various levels to empower persons with disabilities and to ensure their full social inclusion in all areas of life (education, employment, leisure, health and rehabilitation, etc.). People with disabilities must enjoy their rights on an equal basis with others. This shift involves adopting a person-centered and human-rights-based approach in providing the required support, accessibility and accommodations. Professional support through all sorts of services for persons with disabilities must be provided, promoting inclusion and mainstream community-based services, and involving disabled persons organizations in leading the change. In this article, we call “intervention” any effort to support people with disabilities in any domain of life. This may be, for example, an early childhood intervention that empowers families and main caregivers in creating an environment that supports children’s development and acquisition of autonomy, or it may be a specific service that supports a person with disabilities to enter the open labor market by training and supporting the person and by raising awareness of the employer and providing guidance to reasonable accommodation. Several innovative interventions have been developed locally by collaboration among professionals, managers of service providers or mainstream community-based services, persons with disabilities, and their families [1]. These interventions align with the principles of the UNCRPD, as they ensure the social inclusion of persons with disabilities, as well as their autonomy. We call them “effective interventions”. However, they are often place-based and context-dependent and are not easily adapted for use in other contexts. This limitation hinders their wider application. The transformation recommended by the Convention is only slowly being implemented, and there is an urging need to accelerate it [2]. Transferring interventions from one context to another can facilitate the wider adoption of effective interventions and expedite the system transformation needed to drive systematic changes.

Disability is associated with multidimensional and complex experiences [3]. Over recent decades, there has been a significant shift from a biomedical model to a social model of disability that emphasizes how society creates disability and how it can be mitigated by addressing environmental barriers [4]. The International Classification of Functioning, Disability, and Health and the Disability Creation Model provide two convergent conceptual frameworks of disability [5,6,7]. With origins dating back to the emergence of the social model of disability, the Disability Creation Process [6] is based on the interaction of three conceptual domains: personal factors, environmental factors, and life habits. It focuses on how these three domains and their interplay affect the dynamics of systems. This model offers a comprehensive understanding of disability by placing it within a broader interactionist framework that recognizes humans as outcomes of an ongoing process involving biological, physical, and cultural systems. Disability is defined locally and culturally. Life habits are complex constructs that are inseparable from the cultural context, including the meanings and representations associated with them within specific times and places. Identity, one of the dimensions of personal factors, is mediated between individual and societal dynamics. There is a need to incorporate multi-level perspectives in the environmental dimension, encompassing various factors that determine a society’s organization and context in relation to individuals with disabilities. This perspective necessitates interventions at different spatial and intervention levels. Finally, the model acknowledges the dynamic dimension of disability. As a result, disability is portrayed as a dynamic interaction over time among personal, environmental, and lifestyle factors. The Disability Creation Process model emphasizes the complexity and holistic nature of disability. Consequently, interventions in the field of disability are dynamic, complex, and specific to the local context in which they are developed, including sociocultural, socioeconomic, geographical, legal, political, and ethical dimensions. Effective disability interventions often require collaborations across disciplines and sectors, encompassing a wide range of domains, education, employment, recreation, transportation, housing, income, and health [8]. They often target deep cultural change at the individual behavioral, organization, and system levels. In line with the Disability Creation Process model and given the complexity and interactions of interventions with various systems, any transfer of interventions from one context to another must integrate multiple dimensions.

Supporting people with disabilities is considered part of complex interventions. According to the International Classification of Health Interventions, “a health intervention is an act performed for, with, or on behalf of a person or population whose purpose is to assess, improve, maintain, promote, or modify health, functioning, or health conditions” [9]. The Medical Research Council [10] states that “the complexity of an intervention lies in the number of components that act both independently and interdependent, the number and difficulty of behaviors required by those who provide and receive intervention, number and variability of outcomes, number of groups and levels target organizations by the intervention, the degree of flexibility or adaptability of the intervention.” As mentioned earlier, the characteristics of support interventions, including their dynamic nature, multi-level perspective, cultural aspects, and interactions with the environment, align with the idea that supporting people with disabilities falls within the scope of complex interventions.

We found no relevant methodological literature related to the transfer of interventions in the disability field. On the contrary, complex interventions have been extensively studied in the field of public health, encompassing population health, health promotion, and health prevention. Understanding how these interventions can be applied elsewhere is vital for informing policy and practice, which is a concern at the heart of Implementation Science [11,12]. To evaluate complex interventions, the primary question for decades has been: Does it produce favorable results? Transferability, rooted in the science of solutions [13], shifts the focus toward more pragmatic evaluative inquiries, such as why interventions succeed or fail and in which contexts, how interventions meaningfully contribute to observed outcomes, and under what circumstances and for whom [14]. Transferability is defined as the assessment of the extent to which positive outcomes of a successful health intervention evaluated in the primary context can be replicated in the target context [15]. To advance our understanding of transferability, several studies have determined the influencing criteria. Schloemer et al. [16] explored the potential implications of such criteria, considering them facilitators or barriers to transferability. Additionally, a theoretical model for assessing transferability has been proposed. Transferability recognizes the crucial role of context and its influence on intervention outcomes. The effects of an intervention result both from the intervention itself and the context in which it was developed [15]. This question gains particular relevance in the context of complex interventions due to the multitude of contextual factors and their potential interactions, which can modify the intervention and its outcomes [17,18].

Complex health interventions are frequently characterized by their sensitivity to context. However, understanding the context is often clinically oriented and restricted to immediate settings, organizational contexts, or even individual behavior. This approach is rooted in the longstanding history of evaluation anchored in medical science, where context is considered a source of noise or barrier to implementation [15,19]. It appears crucial to broaden our considerations well beyond immediate contextual factors to encompass ethical, socioeconomic, and epidemiological contexts, as well as broader societal processes, which significantly influence the effectiveness of interventions [19,20]. However, by extending the scope of contextual consideration to a more comprehensive overview, dealing with context becomes a challenging endeavor that requires innovative ways of capturing it appropriately. Hawe et al. [21] adopted a systemic strategy when approaching interventions, viewing them as “events in systems.” Although taking into account that complexity has significant implications for how we evaluate transferability, it seems appropriate due to the need to focus on the contextual characteristics of interventions.

To understand the conditions surrounding the transferability of disability interventions, we summarize the latest developments in the field of public health regarding the transferability of complex interventions. The objective was to determine how to operationally support the transfer of interventions. To achieve this, we conducted a comprehensive literature review related to transferability, the results of which will be discussed with regard to their own transferability to the disability field.

2. Materials and Methods

We performed a review to summarize the concept of transferability comprehensively. Although previous studies have attempted to explore the concepts in this field, the terminology lacks clear definitions, and various concepts often overlap or encompass different perspectives. Our aim was to explore this topic comprehensively, including diverse studies that use different terms and employ various research methods, elucidate and map key concepts, identify the methods used, and determine the primary issues. Therefore, we conducted a review with the following objectives: to understand and conceptualize transferability, to examine how transferability is addressed, to operationalize transferability assessment, to explore the involvement of stakeholders in the transferability assessment process, and to investigate how transferability is reported. The protocol for this review was not registered.

Schloemer et al. [16] reviewed articles that describe transferability criteria for health interventions published until 2016. Building upon this work, we reviewed articles published between 2017 and October 2022. One limitation of articles addressing the concept of transferability, like Schloemer’s one, is the exclusive focus on the term “transferability”. However, despite efforts to provide definitions of terms, studies have often used alternative terms interchangeably, such as generalizability/generalization, applicability, and adaptation [15]. Our search queries encompassed these related terms. Recognizing the crucial role of context in the field of transferability, we incorporated this term into our search strategy. Furthermore, given the significant interest in comprehensively viewing interventions in the field of disability with a holistic perspective, as opposed to a narrower organizational, individual, or local viewpoint, we searched for studies that adopted a complexity perspective. Although we mainly focused on primary research, the concept of transferability is also used in systematic reviews with similar characteristics, particularly focusing on the transfer of effective interventions to new contexts, often within a complexity perspective. Therefore, we expanded our search to include systematic reviews. However, although applications of systems thinking in public health are gradually emerging, few systematic reviews have incorporated this into their review processes. We developed a search strategy through an iterative process in collaboration with information specialists to achieve balance between sensitivity and specificity and ensure that the search would retrieve pre-identified eligible studies while also obtaining a manageable number of studies for screening. We systematically searched PubMed, Web of Science, and PsycINFO. The search strategy for PubMed is provided in Supplementary File S1.

This review included reviews, or conceptual or methodological papers based on reviews, that focused on the transferability of complex interventions in public health, published in any language between 2017 and October 2022. We excluded articles related to the COVID-19 pandemic or economic, cost, climate change, clinical, genetic, or epigenetic aspects and those that did not consider transferability as defined or a complexity perspective; addressed a single intervention or subset of interventions; did not relate to an intervention or public health interventions; or were not reviews or methodological or conceptual papers based on a review. The title and abstract of articles were screened by an investigator (ES). Of these articles, 20% were also screened by another investigator (LC) to confirm the selection accuracy. Disagreements between the investigators were resolved through discussion and modifications to the inclusion and exclusion criteria. Next, full-text versions of articles selected based on the title and abstract review were reviewed by ES.

A thematic analysis grid was developed following the research questions and applied for full-text articles. This included the general characteristics, terms used to describe transferability issues, handling of transferability, operationalization of transferability assessment, involvement of stakeholders in transferability assessment, and reporting of transferability.

3. Results

3.1. Articles Selection

The database search identified 418 articles. After eliminating duplicates, 341 articles remained. Of these, 289 articles were excluded based on abstract review, and the full-text versions of the remaining 52 articles were reviewed. Ultimately, 12 articles fulfilled the eligibility criteria. Figure 1 presents the selection process of articles. Two included articles each referred to an article that was a continuation of work performed subsequently or simultaneously by the same investigators. These two articles were included in the final article corpus [22,23]. Thus, 14 articles were analyzed.

3.2. Description of the Included Articles

The first authors of the included articles were from Australia [24], Europe [16,23,25,26,27,28,29,30,31,32], and the United States [22,33,34]. Of the fourteen selected articles, eight were primary research [16,22,24,28,29,31,33,34], six were systematic reviews [23,25,26,27,30,32], nine were conceptualization or methodological papers [24,25,26,27,28,29,30,32,34], and five were reviews [16,22,23,31,33]. Seven articles proposed a framework, tool, or model [22,23,25,26,27,30,32].

Table 1. Characteristics of the included articles.

Title	First Author	Year	Primary Research or Systematic Review Methodology	Type of Research	Concept Mobilized
Taking account of context in systematic reviews and guidelines considering a complexity perspective [25]	Booth, A.	2019	Systematic review methodology	Conceptual/ methodological article	Complexity perspective
A systematic review of adaptations of evidence-based public health interventions globally [33]	Escoffery, C.	2018	Primary research	Review	Adaptation
A scoping study of frameworks for adapting public health evidence-based interventions [22]	Escoffery, C.	2019	Primary research	Review	Adaptation
Interaction of theory and practice to assess external validity [34]	Leviton, L.	2017	Primary research	Conceptual/ methodological article	External validity
Adapting evidence-informed complex population health interventions for new contexts: a systematic review of guidance [31]	Movsisyan, A.	2019	Primary research	Review	Adaptation
When complexity matters: a step-by-step guide to incorporating a complexity perspective in guideline development for public health and health system interventions [30]	Movsisyan, A.	2020	Systematic review methodology	Conceptual/ methodological article	Complexity perspective
The TRANSFER Approach for assessing the transferability of systematic review findings [27]	Munthe-Kaas, H.	2020	Systematic review methodology	Conceptual/ methodological article	Transferability
Systematic mapping of checklists for assessing transferability [23]	Munthe-Kaas, H.	2019	Systematic review methodology/guideline developments	Review	Transferability
La complexité: concept et enjeux pour les interventions de santé publique [29]	Pagani, V.	2017	Primary research	Conceptual/ methodological article	Complexity perspective
Implications of a complexity perspective for systematic reviews and guideline development in health decision-making [26]	Petticrew, M.	2019	Systematic review methodology	Conceptual/ methodological article	Complexity perspective
Making sense of complexity in context and implementation: the Context and Implementation of Complex Interventions (CICI) framework [32]	Pfadenhauer, L.	2017	Systematic review methodology	Conceptual/ methodological article	Implementation
Implementation of evidence-based health promotion and disease prevention interventions: theoretical and practical implications of the concept of transferability for decision-making and the transfer process [28]	Schloemer, T.	2021	Primary research	Conceptual/ methodological article	Transferability
Criteria for evaluating transferability of health interventions: a systematic review and thematic synthesis [16]	Schloemer, T.	2018	Primary research	Review	Transferability
Evidence suggests a need to rethink social capital and social capital interventions [24]	Shiell, A.	2020	Primary research	Conceptual/ methodological article	Complexity perspective

describes the article characteristics, including the authors, publication year, title, research type, and concepts used.

3.3. Concept and Terms Related to Transferability

Numerous terms referring to different concepts were used in the articles, including complexity perspective [24,25,26,29,30], transferability [16,23,27,28], adaptation [22,31,33], implementation [32], and external validity [34].

Transferability was defined in four articles [16,23,27,28] as the extent to which the outcomes of a successful health intervention evaluated in a primary context can be achieved in the target context [15], the definition we adopted (

Table 2. Concepts and definitions used in the included articles.

Article	Concept Mobilized	Definition/Characteristics
Schloemer et al., 2018, 2021 [16,28]	Transferability	Def: the extent to which an intervention whose effectiveness was established in a primary context is effective in a target context.
Munthe-Kaas et al., 2019, 2020 [23,27]	Transferability	Def: assessment of the degree to which the context of the review question and the context of studies contributing data to the review finding differ according to a priori identified characteristics (transfer factors).
Pfadenhauer et al., 2017 [32]	Implementation	No formal definition given but the concept shares the main characteristics of transferability (effectiveness, context): “how contextual factors exert their influence, and how this influence affects implementation success and, ultimately, intervention effectiveness”.
Leviton et al., 2017 [34]	External validity	Def: external validity acknowledges the fact of variation, which increases uncertainty for both decision makers and practitioners about where an intervention will be effective, for whom, and in what context. Share the main characteristics of transferability.
Escoffery et al., 2018 [33]	Adaptation	No formal definition given but the concept shares the common characteristics of transferability.
Escoffery et al., 2019 [22]	Adaptation	Def: modifying a program to meet the needs of the target population, local circumstances, or new contexts. Shares common characteristics with transferability.
Movsisyan et al., 2019 [31]	Adaptation	Def: a systematically planned and proactive process of modification with the aim to fit the intervention into a new context and enhance its acceptability. Shares common characteristics with transferability.
Booth et al., 2019 [25] Movsisyan et al., 2020 [30] Pagani et al., 2017 [29] Petticrew et al., 2019 [26] Shiell et al., 2020 [24]	Complexity perspective	How to handle a complexity perspective in systematic review methodology or complex intervention evaluation? Context is integrated in the concept of complexity perspective. Effectiveness is taken into account in the systematic review and evaluation.

). Two articles that used the terms implementation or external validity [32,34] focused on context and effectiveness and shared the main characteristics of transferability. Three articles used the term adaptation [22,31,33] and focused on how interventions can be modified to produce the desired effects in a new context. The use of adaptation combined with concepts of effectiveness and context is in line with our definition of transferability (Table 2). Five articles related to complex systems thinking [24,25,26,29,30] explored the complexity perspective in a systematic review or primary research with a focus on effectiveness and context, which share common characteristics with transferability.

It is underlined that most papers punctually cited, in addition to the main concept they mobilized, other terms such as implementation, adaptation, transferability, adoption, and translation. This may explain why we retrieved articles dealing with external validity or implementation although these terms were not included in the search keywords.

3.4. How Is Transferability Handled?

3.4.1. Two Main Approaches to Describe Transferability in the Included Articles

Height articles primarily focused on transferability criteria, which influence the outcomes of an intervention [24,25,26,29,30]. Three of these articles summarized and categorized these transferability criteria by reviewing existing frameworks and tools [16,23,32]. Pfadenhauer et al. [32] focused on implementation and context, Munthe-Kaas et al. [23] focused on transferability in systematic reviews, and Schloemer et al. [16] focused on transferability in primary research. These criteria were categorized in domains and subdomains that varied with the focus of the study. Five articles that took a complexity perspective [24,25,26,29,30] conceptualized intervention outcomes as contingent upon contextual interactions within complex systems. Therefore, contextual factors are considered transferability criteria. Conversely, three articles that addressed adaptations [22,31,33] adopted a slightly different approach by examining adaptations that were made or were to be made in interventions. Escoffery et al. [22] reviewed the published adaptation frameworks, Movsisyan et al. [31] summarized the currently available guidelines on adapting interventions to new contexts, and Escoffery et al. [33] explored how adaptation occurs in practice by reviewing published adaptations of interventions. These studies used various taxonomies to classify adaptations, including reasons for making adaptations, types of modifications (additions, deletions, and modifications), and the content of adaptations (i.e., intervention, context, delivery).

3.4.2. Role of Context in Transferability Criteria

The importance of considering context in the study of transferability is strongly emphasized. However, terminology regarding context varies among studies. In three articles that categorized transferability criteria [16,23,32], context was referred to as “context” [32], “environment” [16], or a combination of environment and implementation [23]. Five articles that approached the topic from a complexity perspective emphasized the crucial role of context [24,25,26,29,30]. They asserted that context is often treated inadequately [24,25,30]. Current tools or frameworks tend to define context narrowly, focusing solely on the immediate physical setting in which the intervention is delivered or targeting a limited set of health outcomes and safety considerations. However, contextual features require a more comprehensive understanding encompassing epidemiological, sociocultural, socioeconomic, ethical, legal, and political factors [24,25,26,29,30]. These articles adopt a multilevel perspective, considering the context at micro, meso, and macro levels.

3.5. How Is Transferability Assessed?

3.5.1. Identifying Transferability Factors

Several of the included papers reviewed existing tools or frameworks [16,22,23,25,30,32]. One study evaluated the strengths and weakness of existing tools [25]. Some proposed a new tool [23], research questions [26], conceptual frameworks [16,32], or structured guidance [27,30,31]. Two studies introduced a conversation guide [27,32]. However, a significant challenge arises from the limitless nature of the factors that influence transferability [25,30,34]. Tools designed to identify these factors can become increasingly expansive and, by their very nature, impractical, time-consuming, and resource-intensive [25]. The primary methodological challenge is to prioritize which elements from this expansive array will be most informative [25,26,32,34]. Previous studies have proposed various strategies to address this prioritization issue. Pfadenhauer et al. [32] recommended using frameworks to systematically consider all dimensions of a model and, when necessary, to add granularity within specific domains. Petticrew et al. [26] introduced the concept of the “value of information,” which involves describing the expected value of new information (i.e., does it reduce uncertainty?) that would be generated by conducting new research. This approach allows for a focus on collecting strategic knowledge for decision-making. Consulting stakeholders is another strategy described to tackle this challenge (see below).

3.5.2. An Emerging Topic: Understanding the Underlying Mechanisms

Several of the papers highlight limitations in examining transferability factors and in the effectiveness of existing frameworks in supporting transferability [25,29,31]. Booth [25] suggested that frameworks often tend to artificially separate intervention characteristics from contextual considerations. Several authors have emphasized the need for more critical reflection on creating a coherent understanding of complex interventions and understanding how they function [24,25,29,31,33,34]. This refers to the underlying theory of how the intervention is intended to work and identifying the underlying mechanisms generating the effects of the intervention [29,31,33]. These mechanisms are crucial to the replication of effects and should be transferred. Furthermore, they must be aligned with each specific context [24].

3.6. Consultation of Stakeholders to Assess Transferability

Several papers have recommended involving stakeholders [25,26,27,28,30,32,34] in the process of transfer. Some of the tools developed in those studies assign an important role to stakeholders. In particular, two articles present a conversation guide [27,32]. However, despite this recommendation, stakeholders are often underutilized in practice [27]. Local actors possess valuable practical knowledge about their practice context [27,28]. This practical knowledge plays a crucial role in an inductive process that brings forth relevant topics and contextual factors [25,34]. Another essential aspect of this practical knowledge is that stakeholders help set priorities [26,30,34] by indicating what knowledge is most relevant and impactful. Notably, only three articles provided a definition of stakeholders [27,30,31]. The definitions are broad and include anyone with an interest in, who is affected by, or who is implicated in the findings or the intervention.

Movsisyan et al. [31] categorized the most commonly reported stakeholders into four main groups: (i) representatives of the target population, local partners and organisations, (ii) practitioners, (iii) intervention developers, (iv) researchers and external experts in the field. It has been recognized that these different groups likely have different perspectives and priorities which need to be addressed [26]. Additionally, Shiell et al. [24] introduced the concept of boundaries and the inherent questions of power. Boundaries define the scope of the situation under consideration, delineating what is included or excluded. These boundaries play a significant role in how a problem is defined and influence the range of potential solutions that can be considered.

3.7. Insufficient Reporting

Insufficient reporting is a concern raised by several of the included articles [22,25,27,28,31,32,33,34]. Some investigators have emphasized the importance of establishing detailed reporting standards and templates [16,25,28,34]. For example, Chambers et al. [35] recommended the development of an “adaptome” to catalog adapted programs and their outcomes.

4. Discussion

4.1. Recent Advances in Transferability in Public Health

4.1.1. Defining Transferability and Related Concepts: A Framework

Our review presents recent advances in transferability in public health. It suggests that concepts and terms related to transferring an intervention from one context to another continue to be used interchangeably. Based on our results, we can propose that an intervention in view of transferring it is characterized by its effectiveness, the importance of the context in which it is embedded, and the crucial role of stakeholders. We could distinguish two main approaches to describe transferability: generation of knowledge (through transferability criteria) and concrete actions (through adaptations made or to be made).

There is a clear need for clarification and alignment of the various concepts employed in the field. Based on our analysis and in pursuit of addressing the main question of transferring a successful intervention from one context to another, we can formulate propositions to provide more concrete guidance. Figure 2 illustrates the operational framework to transfer interventions.

The term “transfer” refers to the entire process of transferring a successful intervention from its primary context to a target context. This process involves the management and generation of knowledge with practical actions. The concepts of effectiveness (successful outcomes) and context (primary and target) are closely linked with the concepts of transfer and transferability. Additionally, stakeholders play a pivotal role in facilitating this process.

The terms “implementation” and “adaptation” represent proactive steps in the process of transferring an intervention. Implementation refers to the action that leads to the presence of an intervention in a new context, whereas adaptation refers to the proactive process of modification aimed at aligning the intervention with the new context. These two actions are complementary, and their outcomes result in the development of an intervention that is adapted to the target context.

The term “transferability” refers to the generation of valuable and practical knowledge and evidence that can guide decision-making, support actions, and reduce uncertainty. It represents a transversal process that can be enriched throughout the transfer. It forms the foundation of information that guides a specific transfer and benefits future transfers.

The concept of transferability is primarily guided by two key attributes: effectiveness and context. These attributes should guide the assessment questions. Transferability should be assessed throughout the transfer process. Assessment questions can evolve as the transfer progresses. During the initial assessment, the focus is on anticipation and developing hypotheses regarding transferability, which relies on gathering information related to the intervention in the primary context and understanding the characteristics of the target context. At this stage, the evaluative questions to be considered include the following: How does the intervention work? What are the critical transferability factors? What are the conditions that can influence the outcomes? Is it justifiable to proceed with the transfer? What adaptations are necessary? Toward the end of the transfer, the late assessment phase involves obtaining information related to the adapted intervention, its effectiveness, and the conditions under which it is effective. This assessment validates or refutes the hypotheses formed during the initial assessment and contributes to the enrichment of the transferability assessment. Notably, late assessment is also vital to gain a comprehensive understanding of transferability, as the implementation process can influence how the intervention works in the target context.

4.1.2. Assessing Transferability

Considerable effort has been dedicated to identifying transferability factors. However, this approach has two primary limitations. First, how can we effectively address the nearly infinite number of factors influencing transferability? Second, understanding transferability factors does not enable the generation of adequate actionable transferable data. After investigating the factors affecting transferability, we need to consider what steps should be taken regarding it, how it should be managed, and in what ways it can be beneficial.

Our review indicates that a more comprehensive approach to understanding transferability involves shifting the focus away from transferability factors and towards comprehending mechanisms of change and their interactions with the context. This perspective aligns with the views expressed by Burchett et al. [36] in an examination of the usability and relevance of assessment tools and frameworks. Recent publications [37,38,39,40] have also supported this evaluation approach for complex interventions. In our analysis, the term “mechanisms” emerged from various papers but was not explicitly defined. Cambon et al. [41] demonstrated that different definitions converge on a common point and that mechanisms are prerequisites for change. Mechanisms are considered pivotal functions, and their integrity is crucial to ensuring the transferability of an intervention [41]. When transferring interventions to different settings, variation in implementation, population characteristics, and organizational factors can occur, potentially leading to mechanisms that differ from the expected ones. However, if these adaptations lead to the manifestation of the expected mechanisms, the intervention process can be adjusted to suit each new context. This underscores the importance of mechanisms as key functions that must be replicated to ensure successful transferability.

We then must consider how we can elucidate the mechanisms of change to assess transferability effectively. Some papers have proposed a theoretical approach to understanding how the intervention operates. Recent research [40,41] supports this approach, employing the theory-driven evaluation paradigm. Cambon et al. [41] developed a context-dependent theory-based framework referred to as Intervention System Theory to provide guidance to researchers. This framework enables the assessment of the conditions necessary for change, aligning with the concept of effectuality assessment, which emphasizes context-related effectiveness [42], likely more suitable than traditional effectiveness evaluation.

Our findings also emphasize the significance of stakeholders in assessing transferability of an intervention. They play a crucial role in generating knowledge, encompassing diverse contexts, and prioritizing valuable information. This perspective finds support in several papers [37,38,41]. Furthermore, stakeholders are recognized as contextual elements, given that their positions, preferences, interactions, thought processes, and behaviors can influence the outcomes of an intervention [38,43].

4.1.3. Enriching the Knowledge Base: Reporting Transfers

The information obtained in the late transferability assessment significantly enhances the knowledge base that constitutes transferability and aids the comprehension and documentation of the conditions under which an intervention functions effectively in a target context. As more transfers and adaptations are reported, the body of evidence becomes richer and more informative. Transferability assessments serve as the basis for providing the knowledge necessary to produce implementation and adaptation plans. If previous transfers of the same intervention have occurred, the transfer to a new context can benefit from the information and transferability assessments gathered in prior transfers across diverse contexts. Then, the initial transferability assessment will rely not only on information from the primary intervention and the target context but also on insights gained from the late assessment of adapted interventions. This helps reduce uncertainty in subsequent transfers. Articles that have examined adaptations and adapted interventions carried out in practice, such as Escoffery et al. [33], are particularly valuable in this context. However, it is essential to recognize that this process is ongoing, and no assessment can completely eliminate all uncertainties associated with a complex intervention [37]. Moreover, the knowledge acquired from adapted interventions and future transfers may have implications for the primary intervention, potentially creating a feedback loop. A challenge is to find effective ways to synthesize the lessons learned when interventions are transferred between different locations. The lack of reporting from the field, as shown in our results, is a significant barrier to assessing transferability. Addressing this gap should be a top priority, and a collective commitment to sharing data is crucial for generating useful collective knowledge. The structure of the system should facilitate and promote this shift in practices. Transferability assessment aligns with and contributes to the principles of evidence-based decision-making. Service providers often encounter challenges in implementing evidence-based interventions, with one major barrier being the lack of clear and sufficient guidelines on how to adapt interventions [44]. Addressing these challenges is essential for effective implementation and successful transfer of interventions across contexts.

4.2. Implications for the Field of Disability

Our review has implications for the field of disability. Three issues emerge from our analysis that require further consideration in the context of disability.

4.2.1. Generation of Knowledge: A Pragmatic Approach

First, generation of knowledge should be considered in the field of disability as a pragmatic approach. As shown in Figure 2, transferability involves the process of building knowledge and evidence from primary and adapted interventions. However, specific characteristics within the disability field have notable implications for this statement. (i) Our results show that the lack of reporting in the public health sector is a barrier to transfer. Lack of data in the disability field is particularly concerning [8,45,46]. As said in the Introduction, we found almost no literature related to the transfer of interventions in the disability field. Second, interventions in this field are often motivated by personal experiences and unmet needs [47]. They typically transition from an idea to action with minimal resources, largely due to the extraordinary dedication and energy of people with disabilities, their families, and collaboration with professionals [1,48]. As a result, support interventions often lack a sound methodological basis and may be inadequately formalized, leading to a lack of evidence base in most cases [8]. Given this landscape, advances in transferability within the field of disability require maintaining realistic and pragmatic expectations regarding the resource and time constraints. Prioritizing the acquisition of necessary evidence for transferability is crucial for addressing this challenge. The present review highlights some strategies identified in the included articles. Moreover, this issue raises fundamental ethical and political questions [49]. It involves societal choices and values, placing the disabled persons at the center of societal concerns with regard to their ability to act and make decisions. Therefore, we recommend placing criteria related to the freedom of choice, demand, and the autonomy of disabled people, particularly their feelings, at the core of the evaluation of support interventions dedicated to disabled people. As an example of an assessment framework grounded in ethical concerns, in the realm of health, the new evidence-to-decision framework for supporting guideline development from a complexity perspective is rooted in the WHO norms and values [50].

Addressing this challenge can be approached at both the organizational level of entities providing interventions and within the disability sector as a whole. To bolster these efforts, strategies should be devised to establish robust policies for the evaluation of interventions, promote research, facilitate knowledge transfer, and secure access to funding. This comprehensive approach will contribute significantly to advancing the field and improving outcomes for individuals with disabilities.

4.2.2. Taxonomy of Mechanisms Underlying Change and Core Activities

The second topic deals with the necessity to determine mechanisms underlying change and core activities that ensure that the interventions are effective, i.e., aligned with the UNCRPD principles. Our review underscores that, in addition to considering the factors influencing transferability, it is essential to delve into the mechanisms of change. As stated in the introduction, the evolving paradigm in the disability field (towards the social model of disability) seeks to empower individuals with disabilities and ensure their full social participation. It challenges conventional approaches and attitudes toward supporting them. We can hypothesize that a finite number of effect mechanisms embedded within disability interventions would be best suited to address the specific principles and concepts outlined in the UNCRPD, such as self-determination, empowerment, autonomy, and social participation. We argue that different interventions, even across various disability types and sectors, have a common underlying objective to promote autonomy and social participation and may share common mechanisms of change. This recognition highlights the potential for identifying and applying universal mechanisms within diverse intervention contexts to achieve these fundamental goals.

Following this approach, it becomes feasible to construct a taxonomy of core mechanisms that are essential to ensure interventions align with the principles outlined in the UNCRPD. Additionally, this taxonomy can encompass the activities required to activate these mechanisms. When developing or transferring a particular effective intervention, one can identify and adapt the appropriate mechanisms of effect through various activities to suit the intervention’s characteristics, target population, and the specific context in which it will be implemented. An illustrative example of this approach comes from the field of mental health, where efforts are made to identify common mechanisms for translating mental health interventions into non-specialty settings [51]. The established taxonomy in this context can serve as a valuable reference for numerous other interventions, as mainstreaming disability considerations in interventions is a priority highlighted by the UNCRPD. Another example arises from the study of transforming mental health services toward a recovery orientation. Exploring mechanisms of effect in this context can offer insights into how to facilitate such a transition, with potential benefits for various intervention efforts. This underscores the broader applicability and value of investigating mechanisms of change in diverse fields and settings.

The taxonomic approach suggests that interventions designed for individuals with disabilities should not be studied in an isolated, segmented, disability-specific manner by type of disability, target population, or target sector. Instead, they should be grouped according to the underlying concept or principle they aim to address. Taking a nonspecific disability-related approach would allow researchers to deal with the vast variation in contexts more effectively. This approach would help build a substantial foundation of transferability information and facilitate the operationalization of principles enshrined in the UNCRPD, making it easier to apply successful strategies across a wide range of settings and conditions.

4.2.3. Involving People with Disability in Transferability

The third topic to consider in the field of disability is the involvement of people with disabilities in the process of transferability assessment. People with disabilities (and their representative organizations) are one of the stakeholders’ categories found in the present analysis. Involving disabled persons organizations in leading the change is one key principle of the UNCRPD. First, we argue that disability should follow the enquiry of boundaries as described by Shiell et al. [24]. This approach aims to make explicit the values and perspectives that would otherwise remain implicit within the boundaries. Boundaries tend to be defined by people and organizations with power. In the history of the field of disability, power dynamics have played a significant role [52], and the UNCRPD aims to reinforce human rights. Therefore, it is of utmost importance to bring the perspectives of people with disabilities regarding interventions to light. Various articles have used different methodologies, such as making explicit assumptions (belief systems or subjective experiences). An example is the framework developed by the WHO Mental Health Gap Action Program, which makes explicit assumptions to promote transfer by recognizing differences and diversity [53]. The framework consists of questions that explore key cultural and ethical dimensions of the program. Second, accounting for the experiential knowledge of people with disabilities is essential to address the complexity of real-life situations and to encourage the transfer of appropriate innovations that can be embraced by people [54]. Gardien [55] proposed that experiential knowledge is constructed through daily experiences within specific situations using a set of criteria that encompass both personal and contextual elements. It can be viewed as a means of capturing complexity. Third, beyond the richness of the experiences of people with disabilities as a resource for action, incorporating their experiences places them at the center of their lives. This dynamic represents an opportunity to promote their empowerment [56], autonomy, and full social participation, in line with the developments set out in the UNCRPD.

4.3. Study Limitations

Study limitations: Our study presents some limitations. Because the term “transferability” is not consistently employed in research, and other terms are often used interchangeably, we had to use broad search criteria. Determining when a described concept could be categorized as referring to transferability proved to be conceptually challenging, and it is likely that some articles were inadvertently omitted. Our search for the two primary characteristics, namely effectiveness and context, proved to be a valuable approach. Nevertheless, it is important to note that only four of the included articles explicitly featured the word transferability in their titles or abstracts. This explains why we did not exclusively rely on this term in our article search.

We selected studies with a wide range of designs and objectives, which presented a challenge in terms of synthesis. However, this diversity also serves as a strength of this study, as it combines various approaches and trends. Notably, it integrates the concept of transferability with the emerging complexity perspective, adding depth and richness to the analysis.

5. Conclusions

This review addresses various interconnected approaches for the transfer of complex interventions. It integrates various concepts such as transferability, implementation, adaptation, and complex systems thinking as a new analytical perspective. By synthesizing existing trends and identifying important gaps, it serves to clarify these concepts, making them more tangible. It also underscores the importance of advancing research in this field to generate more practical and transferable data for stakeholders. Finally, our results should guide methodological work on transferability in the field of disability. Future research should investigate whether transferability varies by the type and scale (individual, organizational, or system level) of intervention undertaken or by the nature of the disability. It also should illuminate the place and engagement of operational partners, including persons with disabilities and their representative organizations in the transfer process.