# The Blockchain Oracle Problem in Decentralized Finance--A Multivocal Approach
## 3 Methodology  
In order to answer our two research questions, an appropriate methodology was needed. A classic literature review or systemization of knowledge was, indeed, efficient in the process of summarizing the existing studies [54]. On the other hand, as Sutherland [55] explains, a systematic review should be utilized when specific research questions need to be answered and to ensure the replicability of the results guaranteeing the transparency of the data acquisition [56]. In principle, we opted for the second approach. Two important research databases (Scopus and Web of Science) were then queried to obtain the relevant articles, and specific keywords were utilized within two different research entries. While \"decentralization\" and \"finance\" were inserted in the TITLE-ABS-KEY and Scope, respectively, on Scopus and Web of Science, the \"oracle\" keyword was instead searched in the whole body text. The reason for this difference is that, while decentralized finance should have been the main topic of the article, it is not also requested that oracles constitute the primary area of research. Since our aim is to provide a full picture of contributions posed within the literature, regardless of weight, even a small contribution on oracles or the oracle problem in DeFi is relevant for this research. Table 1 outlines the two research strings for each database. The Scopus and Web of Science databases were both queried on 26 February 2021 and returned 14 and 1 entries, respectively. However, the article returned from the Web of Science database was, unfortunately, off-topic, which means that only articles from Scopus were included. Despite the restricted sample, excluding criteria were implemented to include relevant articles. First, non-English articles were excluded, which reduced the sample to 12 entries. By reading the title and abstract, another five articles were excluded since considered off-topic, despite including the three keywords, which reduced the sample to seven entries. We inspected the article's content to determine whether they discussed oracle or oracle problem issues. We realized that two of the papers were included in the sample for only having references to other articles that include the word \"oracle\" in the title [57; 58]. One article at the end, although using the word \"oracle\" within the text, referred to the Oracle company and not to blockchain oracles [59]. Since oracles were not actually discussed anywhere in the text, we decided to drop the entries, leading to the final sample (displayed in Table 2) of only four papers. While these papers were very informative and well-written, the authors agreed that robust and meaningful results could not be produced based on only four articles. Considering the relevance that Decentralized Finance had in recent months, the lack of scientific papers, which require a considerable amount of time for publication and indexing procedures, is understandable. On the other hand, the web is constantly providing a plethora of articles that could have highly contributed to our research. For that reason, we first decided to include another database (Google Scholar) and further opted for a Multivocal Literature Review (MLR), which is a form of Systematic Literature Review that also includes so-called \"grey literature\" [60]. Whilst MLR includes other non-academic literature, the method followed remains the same as a SLR. Rodney et al. [61] describe MLR as literature \"comprised of all accessible writings on a common, often contemporary topic. The writings embody the views or voices of diverse sets of authors.... They address different aspects of the topic and incorporate different research or non-research logics\". On the other hand, grey literature is composed of documents \"produced on all levels of government, academics, business and industry in print and electronic formats, but which is not under control of commercial publishers\" [62]. Falling under that grouping is information found in news editions, blogs, websites, or whitepapers. The retrieved