# Overview of Blockchain Oracle Research
## 3 Methodology  
An appropriate methodology should be chosen to fulfill the purpose of this study. Furthermore, an in-depth description of the steps followed had to be provided to ensure the reproducibility of the results. A bibliometric analysis was perceived as the appropriate method for reaching the goals of this research. Moreover, its standardized and systematic approach would ensure the reproducibility of results [19; 56]. Building on prior bibliometric analysis [57; 58], the methodology description will first involve database selection, inclusion, and exclusion criteria and, finally, data extraction variables. Regarding data collection, the intention is to include as many articles as possible, as long as they are academic in nature. Therefore, gray literature, such as whitepapers, opinion posts, and news, will not be considered in this research. On the one hand, although not peer-reviewed, this analysis will also consider preprints. The reason for this choice is that the included preprints are written by academics for submission to academic journals. On the other hand, non-peer-reviewed materials, such as opinion posts, are not meant to follow an academic path. Following [PERSON] and [PERSON] [24] and [PERSON] et al. [56], the selected databases were Scopus and Web of Science (WoS), but Google Scholar was also queried. As the analysis also comprises preprints and unpublished manuscripts, limiting the research to Scopus and WoS would not have been a coherent choice. Including a third database would also increase the chance of retrieving other relevant articles. For the three databases, the research was conducted on 2 March 2022. When \"blockchain\" and \"oracle\" were used as keywords in the TITLE-ABS-KEY of Scopus database, 312 articles were identified. In the WoS database, two strings were implemented in the \"Topic\" section so that articles containing the word \"oracles\" were also included and identified. The research returned 143 results. Google Scholar database was queried using the same keywords as those used on the Scopus database, but the queries returned more than 10,000 entries because of their structural differences with Scopus and WoS. For that reason, and due to saturation of results, the author decided to stop the research study on Page 35 (which presents 350 entries organizing results in ten per page). Table 1 summarizes the queried databases, along with the selected research strings. Appropriate exclusion criteria were adopted to narrow down the most appropriate data sample, with the aim of balancing inclusiveness with relevance. However, no restrictions based on language or timeframe were applied because of the nascency of the topic and the research goal. Given that the goal was to gather all relevant information about oracle research, related authors, and institutions, adding a time or language restriction was not a coherent choice. First, the abstract and introduction were read to retrieve and exclude evidently off-topic papers. Many documents were included in the sample for mentioning \"random oracles\" or \"test oracles\", which, despite a similar name, were not the oracles on which this study investigates. Other papers that mention Oracle, the name of a company, were also included, which, although involved in some blockchain projects, is again unrelated to the oracles discussed in this study. After following these steps, 163, 69, and 189 articles were removed from the Scopus, WoS, and Google Scholar samples, respectively. Given that gray literature was also retrieved from the Google Scholar sample, 7 other articles were removed because they were neither written by academics nor published in academic venues. After the duplicates were removed, the three samples were merged, obtaining a nonredundant sample of 282 entries. With the steps mentioned above, the obtained sample was composed of papers that included the \"oracle\" keyword and specifically referred to the communication channels between the blockchain and the real world.