**2. Methods**

This section describes the methodology used to design, implement, and evaluate our data model. Generally, there is a need for a semantic data model to define how data will be structured and related in the database [11], and it is generally acquired that Unified Modeling Language (UML) meets this requirement [12]. Therefore, we first used the formalism of UML [13, 14] to build a conceptual model describing big data of hospital stays. Then, the corresponding rules were used to convert the conceptual model to NoSQL database. In this paper, we choose to focus on document-oriented NoSQL model, namely, MongoDB. This model developed since 2007 by the company of the same name is considered to be the most efficient in terms of performance, for multi-criteria access queries. Finally, the document-oriented model of the big data warehouse was described in JavaScript Object Notation (JSON) format and implemented in the MongoDB database. Subsequently, decision requests were used to evaluate the model.
