## 4 Research Approach  
Addressing the drivers and gaps identified in Sections 1-3, this research proposes a new tool--the SPP Toolbox--to support the development of a Sustainable procurement strategy, oriented towards GPP, SPP and PPI. The SPP Toolbox aims to guide and support public organizations, including public authorities, as they re-think the procurement process, helping them raise their ambitions and broaden their vision, thus changing the organizational approach towards culture, strategies, structures and practices, in the context of the emergence of socio-technical transitions. The approach taken in the SPP Toolbox was adapted from the GPP Management Model, proposed by the European Commission [52] and ICLEI [33; 34], which is comprised of five phases: (1) preparation; (2) target setting; (3) developing an action plan; (4) implementing the action plan; (5) monitoring progress and reporting results. The use of a Deming cycle type of tool offers a simple, flexible and comprehensive approach, facilitating the integration with the existing management systems [53]. Additionally, the SPP Toolbox assembles a range of perspectives, as well as practical skills, concerning the key factors mapped out at the niche level of the STP Model (Figure 1). The toolbox was initially focused on GPP and SPP. It was developed from the existing literature and prototype versions were then tested and applied in three public Portuguese organizations which operate at the local level: Torres Vedras (Municipality of Torres Vedras), Loures (Municipality of Loures) and LIPOR (Intermunicipal Waste Management Company of Greater Porto, Portugal), generating three case studies. The research method was based on multiple case studies, as it allows the researcher to analyze different contexts for each of the cases, i.e., to analyze within each setting and across settings. In a multiple case study, we are examining several cases to understand the similarities and differences between cases [54]. The exploratory case study is developed in line with the grounded theory approach, which assumes that the natural occurrence of social behavior within real-world contexts is best analyzed by deriving \"bottom-up\" grounded categories and concepts [55], i.e., it implies the discovery of theory from data. The research presented in this paper assumes this iterative process of travelling back and forth between the theory and the evidence. The strategy for selecting the cases considered the following criteria: public organizations that illustrated different sizes, organizational structures, purchasing power and with some experience in GPP, SPP or PPI. Willingness to participate was also an important factor that shaped the final selection of cases. Torres Vedras is a medium-sized city, located in a rural area, with 79,500 inhabitants. Its municipality employs 550 people. The municipality of Loures, by contrast, is part of the Lisbon Metropolitan Area and has 200,000 inhabitants and employs about 2100 people. Finally, LIPOR is the inter-municipal waste management enterprise of the Oporto region, employing 191 people. All three organizations have a procurement department but purchasing activities are more centralized in Loures and LIPOR than in Torres Vedras. With reference to their experience in GPP, SPP and PPI, all three organizations had some previous experience but this was limited to GPP. LIPOR, in particular, implemented the SA 8000 social accountability management system and, therefore, has applied a code of conduct for all suppliers and subcontractors, since 2009. The three organizations thus represented different situations and were considered adequate examples for testing the SPP Toolbox. The case studies were implemented in the period 2012-2014, via regular meetings held in the three organizations' premises, with the aim of implementing the six steps of the SPP Toolbox.