*3.2. Research Goals*

The main objective of this study is to find out the major contributions, solved problems, and challenges of IoT and WSN in Industry 4.0. In addition, the research gap and limitations of current work in these areas helped us find room for improvement; we also explored future directions and possible outcomes in this area. We designed questionnaires on these domains to find high quality research papers. Some of them are listed in Table 3. In addition, we answer the research questions in Table 3 and in Section 4 .


### *3.3. Selection of Primary Studies*

It was a challenge to search the specific and limited computer science/engineering databases to ge<sup>t</sup> a "complete picture" of the research questions. After formulating the research questions, we collected research papers from various repositories, such as IEEE Xplore, ACM, Wiley Online Search, Elsevier, etc. The papers from electronic databases with the areas of IoT, WSN, and Industry 4.0 were efficiently evaluated. Figure 5 shows the names of the repositories where the research articles were collected from 2014 to June 2021.

**Figure 5.** Repositories versus number of studies Used.

### *3.4. Selection/Search Criteria*

This SLR combines three domains: IoT, WSN, and Industrial Revolution 4.0, so the search strings were related to each domain and its applications in Industry 4.0. Table 4 shows the subject search strings used to search for relevant articles. The search strings were divided into five groups to search for relevant articles from reputable journals and conferences.



### *3.5. Inclusion and Exclusion Criteria*

Several research articles were found on the mentioned domains, IoT, WSN and IR 4.0. To extract the most relevant and concise data from these articles, the following criteria were chosen, as shown in Table 5. The publicly available papers written in English and related to the search strings and research objectives in IoT, WSN, and IR 4.0 published between 2014 and 2021 were considered for further SLR tracking.

**Table 5.** Inclusion and exclusion criteria.

