My first Publication Agile-Data-Warehouse-Design-eBook | Page 72

Modeling Business Events 51 Figure 2-10 Adding quantities Unit of Measure When you ask the stakeholders for example quantity values, you should also discover their unit of measure. If you find that a quantity is captured in multiple units of measure it will need to be stored in a standard unit in the data warehouse to produce useful additive facts, so you should ask the stakeholders what that standard unit should be. (Chapter 8 provides details on designing additive facts.) You record the unit of measure in the quantity’s column type using square brackets type notation; e.g., [$] or [Kg]. The unit of measure is a more useful descriptive type for a quantity than [how many]. Ask for the standard unit of measure for each quantity [ ]: Square brackets denote detail type (e.g. who, where) and unit of measure for how many details. If a quantity needs to be reported in multiple units of measure you can record them as a list with the standard unit of measure first. Figure 2-10 shows examples events where REVENUE is captured in dollars and pounds. The column type [$, £, €, ¥] records that US Dollar is the standard unit for the data warehouse, but BI applica- tions will also need to report REVENUE in Sterling, Euro and Yen. Durations You discover durations by asking how long questions. For example, “How long does it take a CUSTOMER to order a PRODUCT?” If the stakeholders view the event as a point in time there will be no duration (not recorded or not significant). Asking for durations is another way of testing if the event should be modeled as evolving. Duration calculations can expose missing when details and highlight other events (verbs) that are so closely related to the current event that they should all be part of an evolving event. Multiple units of measure can be listed in the column type Ask “How long?” to discover durations and evolving events