## 3 Materials and Methods
### Planning the SLR Process
#### 3.1.3 Setting the Keyword Plan  
This research aims to figure out the relevance of using both digital technologies and Lean principles in SCM. Therefore, the literature search is conducted based on three levels of keywords. The selection of keywords for each level will be motivated in the next sub-sections. _First-level keyword selection (Supply Chain)_  
The keyword \"supply chain\" was first applied to delimit the search scope. It helps to obtain all the possible results about the context of this SLR. _Second-level keyword selection (Digital technologies)_  
Industry 4.0 as a general concept is characterized by many definitions and includes various digital technologies [22]. Hence, due to the vagueness of selecting the related keywords in the previous works [10; 12; 13; 39], the present SLR aims to provide a reliable basis for the literature searching by identifying the Industry 4.0 technologies most debated in SCM. Selecting relevant keywords for emerging Industry 4.0 technologies in SCM. At the first step, the most relevant keywords are collected from [21; 24; 26; 43; 51; 52; 53; 54; 55]. Due to theearly stage of the research about Industry 4.0 technologies in SCM [40; 42; 56], the first list of keywords is checked during the next phases of the research by the papers obtained from Scopus. However, the comparison confirmed the completeness of the first list of keywords. These fifteen keywords are Augmented Reality (AR), Cloud Computing, Robotics (R), Sensor Technology (ST), Omni Channel (OC), Internet of Things, Self-Driving Vehicles (SDV), Unmanned Aerial Vehicles (UAV), Nanotechnology (N), 3D printing and Additive Manufacturing, Blockchain, Big Data Analytics, Artificial Intelligence (AI), Cyber-Physical Systems (CPS), and Simulation. Only the keywords mentioned more than one time in the reference papers [21; 24; 26; 43; 51; 52; 53; 54; 55] are selected. Thus, the keywords \"Sensors Technology\", \"Nanotechnology\", and \"Simulation\" are identified as non-relevant keywords for searching about digital SCM. Afterward, the remaining keywords are checked with two SCM experts, and, by considering the scope of the research, \"Omni channel\" is removed from the list. Additionally, in order to minimize the possibility of missing relevant works in Scopus, all the possible variants of the keywords in the final list are searched, e.g., both \"Block chain\" and \"Blockchain\". The searching protocol is using a combination of each the selected keywords with Supply chain as the second level keyword (e.g., \"Supply chain\" AND \"Cloud Computing\") in the paper title, abstract, and keywords. Additionally, with the aim of addressing high-quality scientific contributions [57], the search focuses only on journal papers as the main resource for selecting technology keywords. The time for searching these queries is October 2020. Refining the most relevant keywords for Industry 4.0 technologies in SCM. In this step, all the titles, abstracts, and keywords of the English journal papers from the results of the previous step are scrutinized. By doing this step, the main aim is refining the list of the Industry 4.0 technologies to remove the technologies that have no strong relationship with the SCM based on the published English journal papers. Based on the outcomes of reading title, keywords, and abstract, the total average percentage of relevant papers to the SCM (34%) is considered as a criterion to identify the relevant digital technologies. Therefore, by removing the CPS and R the following technologies are considered as technologies to analyze in more detail in the next step: BC, BDA, AM, AR, IoT, SDV, AI, UAV, CC. Identifying the most debated digital technologies in SCM to be used as keywords. As a further step of the analysis to finalize the list of the Industry 4.0 technologies in SCM, the trend of publications on each technology is here investigated. By doing this step, the main goal is completing the results of the previous step by taking look at the trend of publications for each single digital technology.