Measurement units

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Magne Myrtveit
Posts: 57
Joined: Mon Jan 12, 2009 6:52 am

Measurement units

Post by Magne Myrtveit » Sun Aug 25, 2013 2:57 am

Without measurement units, either explicitly specified or implicitly derived from variable names, documentation, and other forms of communication, it is impossible to interpret the meaning of numbers used in, and resulting from, equations.

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
Currently, different tools support units to the levels listed below:
  • Microsoft Excel, (and DYNAMO?) - Level 0
    Vensim and iThink - Level 1
    The SI system - Level 2
    Powersim Studio - Level 3
    Dynaplan Smia - Level 5
My suggestion is that "Basic SD" is defined to have Level 1 unit support, and that the remaining levels of the standard are defined as described here, and used to label models containing units at the higher levels.

Best regards,
Magne

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