## 2 Research Method
### Data Eligibility and Analysis of the Literature  
The article selection procedure uses the PRISMA flow diagram [33], as shown in Figure 2, which outlines papers' search, inclusion, and exclusion. There are three steps in the PRISMA flow diagram: identification, screening, and included. Firstly, in the identification step, we performed a comprehensive literature review between 1 January 2018 and 31 June 2021, using PubMed, Web of Science (WoS), Association of Computing Machinery Digital Library (ACM DL), Science Direct, and IEEEXplore digital libraries. We start from 2018 because we are interested in further implementation in the medical area one year after federated learning was proposed in 2017 [16]. The following search phrases were used in general are \"Federate learning,\" and \"Healthcare,\" and \"data privacy protection.\" Because each publication database has its own set of filters for search queries, the specific query terms are specified in Appendix A Table 1. The initial result from digital libraries showed 197 articles satisfying the search criteria. Then, 28 articles were removed due to duplications, ending with 169 articles in the identification step. While systematic reviews offer various advantages, they are prone to biases that obscure the study's objective results and should be evaluated cautiously [35]. Several approaches were used to eliminate bias and ambiguity in the research selection process, such as (i) conducting a dual review, (ii) defining clear and transparent inclusion and exclusion criteria, and (iii) tracing the resulting flow diagram using the PRISMA flow diagram. Firstly, two researchers independently analyzed the data and resolved inconsistencies through group discussion (P. and K.T.P). Then, the abstracts and complete texts of all relevant articles were carefully studied, and only those that fit the inclusion and exclusion criteria were chosen. Researchers then confirmed the selected papers and resolved any conflicts; if any disagreements persisted, third researchers were invited to discuss the matter, and the findings were appraised (Z.-Y.S., C.-R.S., and W.J.). There was no dispute over the papers included in this review. This study should propose a good overview of FL for the healthcare sector and more in-depth about establishing FL's secure medical data mechanism. Thus, in the screening step, we define the inclusion and exclusion criteria. We included publications that (i) use FL to develop a model on a medical dataset, (ii) are published in well-known journals, and (iii) are published in English. Exclusion criteria were used to exclude the published studies that were not related, based on the following criteria: (i) articles that are not related to FL, (ii) FL for nonmedical application or not using medical dataset in the experiment, (iii) non-English language, (iv) review article, (v) proceeding or conference papers, (vi) arXiv preprints, and (vii) book, book chapter, book section. Numerous considerations exist against the inclusion of conference papers in this study [36]. Firstly, conference proceedings usually contain various topics and much larger set of publications such that identifying suitable conferences, accessing their abstracts, and sifting through the frequent thousands of abstracts can be time-consuming and resource-consuming. Secondly, conference proceedings may lack sufficient information for systematic reviewers to evaluate the methods, risk of bias, and outcomes of the studies submitted  
Figure 2: Study selection using PRISMA flow diagram method consisting of identification step, screening step, and included step. at the conference due to their brevity. Finally, the reliability of the results is also in question especially in the healthcare area, partly because they are frequently preliminary or based on limited investigations undertaken in a position to meet conference deadlines. Thus, we do not include the conference papers in the inclusion criteria.