page_title: Dimensional fact model
section_title: Basic concepts
hierarchical_section_title: Dimensional fact model / Basic concepts
caption_attribution_description: English: A simple fact schema for the invoice fact showing the basic constructs of the Dimensional Fact Model
context_page_description: The dimensional fact model is an ad hoc and graphical formalism specifically devised to support the conceptual modeling phase in a DW project. DFM is extremely intuitive and can be used by analysts and non-technical users as well. A short-term working is sufficient to realize a clear and exhaustive representation of multidimensional concepts. It can be used from the initial DW life-cycle steps, to rapidly devise a conceptual model to share with customers.
  Data warehouses are databases used by decision makers to analyze the status and the development of an organization. DWs are based on large amounts of data integrated from heterogeneous sources into multidimensional databases, and they are optimized for accessing data in a way that comes naturally to human analysts.
  Data in a DW are organized according to the multidimensional model, that hinges on the concepts of fact and dimension. Each fact is quantified through a set of numerical measures, such as the quantity of product sold, the price of products, etc.
  DW design and development require ad hoc methodologies and an appropriate life-cycle.
context_section_description: A fact is a concept relevant to decision-making processes. It typically models a set of events taking place within a company. Examples of facts in the commercial domain are sales, shipments, purchases, and complaints.
  A measure is a numerical property of a fact that describes a quantitative attribute that is relevant to analysis. For example, each sale is measured by the number of units sold, the unit price, and the total receipts.
  A dimension is a property, with a finite domain, that describes an analysis coordinate of the fact. A fact generally has multiple dimensions that define its minimum representation granularity. Typical dimensions for the sales fact are products, stores, and dates; in which case, the basic information that can be represented is product sales in one store in one day.
  A fact is represented by a box that displays the fact name along with the measure names. Small circles represent the dimensions, which are linked to the fact by straight lines (see Figure 1).
  A dimensional attribute is a property, with a finite domain, of a dimension. Like dimensions, a dimensional attribute is represented by a circle. For instance, a product may be described by its type, category, and brand; a customer may be represented by city and nation. The relationships among the dimensional attributes are expressed by hierarchies.
  A hierarchy is a directed tree whose nodes are dimensional attributes and whose arcs model many-to-one associations between dimensional attribute pairs. A hierarchy includes a dimension, positioned at the tree's root, and all of the dimensional attributes that describe it. Arcs are graphically represented by straight lines that connect dimensional attributes. Hierarchies define the way elemental business events can be selected and aggregated for decision-making processes.
caption: Figure 1: a simple fact schema for the invoice fact
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