Defining “scale” and “scaling up”
We found that the definition of the term “scale” in the literature has expanded and evolved in the last 30 years, without an accepted and widely utilized version identified . Earlier definitions of scale are narrow, reflecting quantitative measures of program scope, such as number of sites covered or beneficiaries served [14, 15]. In more recent literature, most scholars view these definitions as insufficient. The CORE group, a collaboration of health professionals from international development NGOs, has proposed a broader conception of “scale”, defined as “widespread achievement of impact at affordable cost”, with “impact” serving as a function of coverage, effectiveness, efficiency, local ownership promoting sustainability, and equity . CORE’s definition closely reflects those Coburn proposed  in the educational arena. Coburn’s framework included sustainability, spread, and shift in ownership, along with a fourth dimension: deep and consequential change.
Similar diversity is found in the scope of what “scaling up” entails. In a literature review on scaling up, Mangham and Hanson  argue, “scaling up is primarily used to describe the ambition or process of expanding the coverage of health interventions, though the term has also referred to increasing the financial, human, and capital resources required to expand coverage.” Although “scaling up” is still primarily employed to describe geographical reach, Uvin and Miller’s 1996 article, “Paths to Scaling Up”, sparked a more sophisticated dialogue by including dimensions beyond quantitative indicators: in addition to quantitative (geographic spread or expansion in size) scale up, they define “functional” scale-up as expanding the scope of activity; “political” scale-up as influencing the political process; and “organizational” scale-up as enhancing organizational capabilities and sustainability .
The World Health Organization’s global network of public health professionals, ExpandNet, proposed that scaling up involves “efforts to increase the impact of health service innovations successfully tested in pilot or experimental projects so as to benefit more people and to foster policy and program development on a lasting basis” . This definition highlights the scientific or technical dimension (proven interventions), the political dimension, and the challenge of sustainability.
The 15 sources we reviewed differed in their approach to studying and explaining scale and the overall intent of the work (descriptive versus normative). Some included frameworks designed to assist program managers, implementers, and other actors in making practical decisions about scaling up. Some sources also provided insight into key capabilities or choices associated with success or failure in achieving scale. Despite the disparate nature of the sources studied, we were able to synthesize domains that consolidated much of the conceptual thinking about scale and scaling up found in the literature.
We identified 10 domains, each common to at least three of these works and included as integral to successful scaling up. While some of the domains were found across most of the reviewed publications, none were explicitly included in all 15. These domains were important to provide a framework from which to explore the next phase of project development: sustaining delivery at scale.
Ensuring adequate , flexible , reliable, and sustainable funding [7, 15, 19]. This can be accomplished by incorporating a program into the national budget [8, 11, 20] or the core budget of the funding agency.
Mobilizing support for the program and protecting it from vested interests that may perceive it as a threat . Obtaining the support of political leadership and champions who ensure sustained, visible, and high-level commitment to the program  at all levels of government and among relevant private-sector actors and civil society organizations .
Community involvement, integration, buy-in, and depth
Striking an appropriate balance between participatory and expert or management-dominated approaches . Grounding scaling up in the principles of respect for and promotion of human rights and in the value of participatory and client-centered approaches . Adapting the program to local contexts  and addressing the community’s identified needs . End users should be engaged early on  and community champions involved in program design, implementation and scale-up . Cultivating the depth of change necessary to support and sustain consequential change .
Ensuring that domestic and external partners either continue or are engaged to support the program . Includes a systemic view of the variety of actors in the broader environment and a strategic understanding of how they can be leveraged to influence the scaling-up process . Determining and ensuring appropriate balance of scaling-up responsibilities—additive (full burden on one organization) or multiplicative (distributed across several organizations) .
Balancing flexibility/adaptability and standardization
Striking an appropriate balance between flexible, adaptive strategies and implementing a standard package of interventions . Ensuring that universally effective components of an intervention are applied while allowing for local adaptation . Evaluating, learning, and changing the approach as scaling up proceeds and developing a culture of adaptation, flexibility, and openness to change . Planning for context-specific delivery mechanisms effective in going to scale .
Supportive policy, regulatory, and legal environment
Ensuring that a supportive policy, regulatory, and legal framework [7, 9, 18] has been developed that allows for scaling up. Inclusion of program in national policies [7, 8, 11].
Building and sustaining strong organizational capacity
Addressing shortcomings in organizational capacity and enhancing the ability to deliver intended services and support [7, 9]. May include building local capacity  and partnering with others able to operate the scaled program . Ensuring staff is sufficient, well distributed, and qualified with strong technical and program management abilities . Strengthening human capacities  in management and implementation within national and sub-national governments .
Shifting ownership so that it is no longer an “external” process controlled by reformers but instead becomes an “internal” process led by local actors with the capacity to sustain, spread, and deepen the results . May include successfully transferring intervention to adopting organizations including to national or local government .
Determining and ensuring the appropriate balance of reach, influence, and resources provided by centralized authorities and local initiative, autonomy, spontaneity, mutual learning, and problem-solving provided by a decentralized approach . Decentralizing management [17, 18] and programmatic activities to the local level .
Ongoing focus on sustainability
Creating a lasting programmatic and policy impact that produces enduring health benefits [7, 8, 11]. Consistently focusing on sustainability  and devising a strategy that includes plans and actions to ensure sustainability. This focus may inform the path chosen to achieve scale. Uvin argues that this decision on path should reflect the nature of the intervention and local environment and may influence ability and need for sustainability .
The reviewed literature also identifies several other decision points in planning the scaling up of public health programs that are likely to inpact sustaining delivery at scale and reflect the complexity of replication of a program. These choices include decisions about when and if to transition to another organization (including the government) and the importance of fidelity to an effective model versus the latitude to adapt to local conditions .
Sustaining delivery at scale domains: lessons learned from HIV/AIDS
While we found a general domain of an “ongoing focus on sustainability”, there were few references to how the other domains were relevant to sustaining delivery at scale. Our review of the 11 case studies showed each of the 10 domains had elements relevant to how these programs worked to sustain delivery at scale. Some of the domains became even more critical as programs transitioned from having achieved scale to aiming for sustaining delivery at scale. For example, for Domain 5 (balancing flexibility/adaptability and standardization), we found that as programs achieve scale, many struggle to balance continued local innovation with standardization under central control. The case studies suggest that program monitoring and evaluation and elements of financial control and oversight, as well as areas where there are economies of scale such as supply chain management and training, can be successfully standardized and contribute to sustainability. Elements of delivery models that will continue to need iterative adaptation include generating local demand and mobilizing communities.
The Avahan Indian AIDS Initiative, a large-scale HIV prevention program of the Bill and Melinda Gates Foundation, for example, used similar metrics and financial management structures across its seven “state lead partners” and 137 district-level NGOs that were implementing HIV prevention programs. But each NGO used a unique set of activities to achieve its goals and reach the local target population. Avahan achieved high levels of success using this model balancing management and monitoring with local adaptation of interventions to reflect targeted populations. To date, Avahan’s model is being evaluated as it transfers its model to the public sector. Further research is required to understand how and where flexibility ensures ongoing impact and where it poses a threat.
While Domain 7 (building and sustaining strong organizational capacity) was critical to sustaining delivery at scale, an even broader long-term approach to capacity was also found to be important. This approach should expand to include a focus on providers and key staff implementing the program by investing in long-term human resource retention and development. For example, a mature program might lose experienced staff if there is little room for professional development or “burnout” from prolonged time working in weak health systems and if there are ample opportunities to attain better-paying positions elsewhere. If programs do not plan for retention and staff turnover, there will be inadequate numbers of appropriately trained new staff (both internally for program management and for implementers as programs transfer to public sector), threatening sustainability of gains.
Therefore, a focus not only on meeting human resource demands as programs scale, but on developing internal human resources management to improve retention and maintenance of program expertise is likely to be essential for sustaining delivery at scale. Organizations featured in the case studies that have successfully sustained delivery at scale have deliberately created a culture of commitment and optimism among their employees to address staff retention [24, 25]. The director of The AIDS Support Organization (TASO), a Ugandan NGO, for example, looked for motivated young people to hire and gave them significant responsibility. The antiretroviral therapy coordinator explained, “[Working at TASO] was very, very exciting, and people were so empowered with hope—'I can do this! This is work that was once restricted to doctors, but now I’m actually saving lives!’” The director valued good management skills over HIV knowledge in hiring and made sure staff members felt that they had a career advancement path within the organization.
The World Health Organization/ExpandNet framework includes equity as an important component in scaling up , but it was not prominent in other sources reviewed. As the vision for scaled programs is increasingly focused on transition to the public sector as the main pathway to sustainability, an emphasis on equity (i.e., a focus on ensuring access that includes the most vulnerable) needs to be an increasingly important driver of decisions and delivery models to ensure that this sector is able to fulfill the role of serving the entire population . Iran’s Triangular Clinic, for example, served HIV-positive individuals from various risk groups, many of whom were injection drug users who had been ostracized from their families and society at large. Assisting all populations affected by HIV, not just those easiest to reach or least stigmatized, initially led to a profound local impact and was later adopted by the national program, scaled up around the country, and integrated with the health care system. We would therefore propose an 11th domain: emphasizing equity.