An EDGE/Evaluation is a set of values (N,U) where N is a number and U is a unit. This concept is deliberately left open to interpretation in order to give flexibility and to allow all manner of systems to use it for their own purposes.
A 'unit' defines a 'unit of account'. Giving different units for 'apples' and 'oranges' prevents them from being added together. Different people will choose different units, so one might lump apples and oranges together and account them as 'fruit', another might wish to distinguish different grades or sizes of apple etc. This is fine since people's evaluations don't have to match.
The syntax to specify an EDGE unit is simply its URI. In practice, this will probably be a resolvable URL as well as a URI (i.e. a resolvable address, e.g. http://... from which people can access the Unit definition document) though this is not mandatory.
Main article: EDGE/Unit definition
Units are specified by an XML document, which when downloaded, allows EDGE software to handle the unit correctly, e.g. by presenting it properly to users. This standard is currently simple, but could be extended to allow for use by more advanced semantic web software.
Use of URLs
Use of units is decentralised, so people are free to create and use new EDGE/units at any time. The question therefore arises about how to handle Evaluations which include an unknown unit. Having a URI which is also a location provides a simple way to learn more about this - e.g. try to receive a document specifying how to handle (e.g. interpret/display) this unit. See for example, the EDGE unit definition document at http://RobinUpton.com/ns/hour .
Choice of Units
While anyone can create units at will, in practice, most users will do only what is easily accessible through a GUI. The most obvious set of units are national currencies: $, £, € etc. Another pair of units of account that may prove popular are nights of accomodation and meals provided. Any physical object which people exchange would likely end up having sets of corresponding units - and later, more advanced software, could establish equivalences between them.
For goods, the most useful units would likely be whatever was used by their friends and friends of friends. EDGE software could automatically collect and share units used by friends' evaluations just as some Email software collects email addresses of people it has received email from. People could create units in areas of their expertise - so, for example, a fruit grower might be unhappy with a unit of "fruit" and instead decide to count "apple"s and "orange"s separately. Another might decide that "apple" was too specific and to account differently for "Granny_Smiths" and "Golden Delicious". A GUI could allow currencies to be created without much effort, while currencies used by friends could be loaded automatically.
For services, a particularly popular unit of account that people may wish to use while evaluating interactions is hours of their time. This is generally understood as a global unit (i.e. by time bankers' as my time = your time = anyone's time) but it would probably make more sense for people to use a separate URI to represent an hour of their time, as distinct from anyone else's time. When giving time, since people can't actually give an hour of anyone else's time, far less an amorphous one-unit-fits-all 'global time', but can only give an hour of their own time.
Since URIs are effectively not human readable (programmers don't count as humans for this purpose :) normal end users should never be shown URIs. Instead signs, icons or (ideally language-specific) summaries, e.g. "$" instead of "http://gifteconomy.org/ns/USD", "Hour of Robin Upton's Time" instead of "http://RobinUpton.com/ns/hour". Presentation information is contained in the Unit definition document.
- http://altruists.org/ae9 Personal Time Banking]