Tips on model design
Here some tips and best practices for models that are easier to build, understand, debug, and extend.
Use constants: Avoid numbers in expressions
In models by novice modelers, you often see variables with definitions that contain numbers like this:
Variable Energy_consumption := Gas_consumption * 239.2972
You may wonder what the number 239.2972 means -- unless you happen to know it is units conversion factor from million BTU to Kilowatt-hours. It's much clearer if you create a separate Constant for that number:
Constant KWH_per_MMBTU := 239.2972 Variable Energy_consumption := Gas_consumption * KWH_per_MMBTU
Using a named Constant like this makes it obvious what the number means. Plus you can use this Constant KWH_per_MMBTU wherever you need it -- without having to look up and retype the number each time.
Remembered to specify the units for each quantity:
Units OF Gas_consumption := 'MMBtu' Units OF Energy_consumption := 'kWh'
If the number is a not a constant, but a model input value, like the population of the city, or a date, it's even more important to put it in its own variable, with a description explaining what it is and where it came from -- rather than embedding it into a definition.
Don't repeat expressions in cells of a table
If you find yourself entering the same or similar expressions into some or all the cells of an edit table -- there's almost certainly a simpler and clearer way to do what you want. Consider this table:
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