Issue #77: Include LIM Variable work

Excerpt from LIM document

  Variables Package

1. The following 5 objects, listed alphabetically, are added to the Variables Package in LIM: Measurement Type, Measurement Unit, Sentinel Value Domain, Substantive Value Domain, Universe. The subsequent changes from GSIM v1.1 are described below:

Universe

 Figure 10: Population, Universe, Unit Type, Concept

1. A new object, Universe, was added (see Figure 10) that supports time trends and geographical comparisons.
2. LIM Definition: A defined class of people, entities, events, or objects, with no specification of time and geography, contextualizing a Unit Type.
3. Population is a specification of Universe.  A Universe is a Concept and can have sub-Universes (parent-child relationship). The Universe itself is a specification of Unit Type.
4. The association between Universe and Unit Type in LIM replaces the association between Population and Unit Type in GSIM. The associations between Population and Unit, Unit and Unit Type is the same as in GSIM.

Variables, Conceptual Domains and Value Domains

Figure 11: Variables, Conceptual Domains and Value Domains

Conceptual Domains

1. Two new attributes are introduced (see Figure 11) for Enumerated and Described Conceptual Domains. Each of these have an attribute sentinel with Value Type Boolean indicating whether they are sentinel (1) or substantive (0).
2. Sentinel values (value used to represent a state in the processing life-cycle e.g., missing data) depend on the processing stage e.g. collection of data in an interview, publication of data after the application of non-disclosure controls. Separating the sentinel values from the substantive ones (value used to represent an observation of some Unit of interest) allows a large reduction in the number of Conceptual Domains, and thus Variables, that need to be maintained.

Sentinel and Substantive Value Domains

1.     Two new objects are introduced (see Figure 11): Substantive Value Domain and Sentinel Value Domain. Each of these can take their values from Enumerated Value Domain or Described Value Domain, depending on whether they are listed or not. Separating the sentinel values (e.g. unspecified) from the substantive ones allows a large reduction in the number of Value Domains, and thus Represented Variables and Instance Variables, that need to be maintained.

Measurement Type and Measurement Unit

Figure 12: Conceptual Domain, Value Domain, Measurement Type and Measurement Unit

1. Two new objects are introduced (see Figure 12): Measurement Type and Measurement Unit.
2. Measurement Unit is a means to characterize quantities by specifying how to interpret a unit of that quantity. For instance, the quantity 2 measured in miles per hour, the quantity 2 measured in meters per second, and the quantity 2 measured in furlongs per fortnight are all different. The Measurement Units determine what each unit of a quantity means. However, as this example shows, some quantities belong to the same class or type. Each of the quantities above measures speed. There are others than measure force, temperature, or currency. All Measurement Units for speed, for example, are equivalent and a transformation exists to translate quantities in one measure to those of another: 1 mi/hr * 5280 ft/mi * 12 in/ft * 2.54 cm/in * 1/100 m/cm * 1/3600 hr/s = 0.44704 m/s. This means that all Measurement Units of the same type form a set of translatable units. This set is called the Measurement Type. Sometimes, one Measurement Unit for a Measurement Type is given the status of a standard. For instance, meters per second may be a standard measure of speed. There is no requirement that each Measurement Type must have a standard Measurement Unit identified. Each Measurement Type has as a standard at most one Measurement Unit.
3. LIM Definition: The Measurement Type defines the type of a measure e.g. mass or currency. The Measurement Type groups all Measurement Units, which can be converted into each other. A Measurement Type can have a standard Measurement Unit, which can be used for conversion between different Measurement Units.
4. LIM Definition: A Measurement Unit is the metric for a measurement in terms of an official unit of measurement.
5. Measurement Units can be based on different Measurement Types such as weight, height, currency, duration etc. Measurement Units can be transformed into one another (e.g. kilometres into metres) if they refer to the same Measurement Type (e.g. length).