Unit support is provided by many of the popular SD tools. In contrast to array support and hierarchy, unit support can be arranged along a linear line of development, starting with no support for units and ending with two extensions that take the unit system beyond physics and into finance.
In 2008 I wrote a blog article on the nature or units, the benefits of units, and the different levels of unit support provided by system dynamics software and spreadsheets. Instead of repeating the article here, I include a link to it: https://www.dynaplan.com/blog.php?page=thread&tid=1616.
For reasons given elsewhere, the objective of a standard for SD should not be to make every system dynamics software equal. But it might be worth while to define a common base, with a feature set close to the minimum level of functionality provided by current SD tools. From there, vendors can evolve their software to higher levels, at their own pace.
The following is a list of the six levels of unit support:
- Level 0: No support for units
Level 1: Base units (atomic units)
Level 2: Level 1 + derived normal units
Level 3: Level 2 + derived point units
Level 4: Level 3 + time to calendar time conversion
Level 5: Level 4 + currency conversion
- Microsoft Excel, (and DYNAMO?) - Level 0
Vensim and iThink - Level 1
The SI system - Level 2
Powersim Studio - Level 3
Dynaplan Smia - Level 5