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units for percentages in Vensim models
Posted: Wed Apr 30, 2003 3:16 pm
by Salima
I have a question about the units taken by percentages (ex.15%); sometimes their units are :
-fraction
-1/unit time (1/year or 1/month,..)
-dimensionless
My question is : do we choose one of these units according to the unit we desire to get for a resulting variable or is there another reason.
Thanks in advance. Salima
Posted: Wed Apr 30, 2003 4:01 pm
by Administrator
If I have a paramater that is, for example, "percentage of population fertile", it is dimensionless. You multiply this by the population to give you the "population that is fertile". So for me, all percentages in my models are dimensionless.
I would not manipulate the units at all to make an equation balance, as if you do this, the equation would most likely be wrong.
Tony.
Posted: Wed Apr 30, 2003 4:32 pm
by Lee Jones
Percentages and fractions are always dimensionless. To avoid errors when using percentages, I suggest using a dimension called "percent". Then, when you come to make use of the percentage, divide by a constant called "100percent" which is =100 with dimensions "percent".
This would avoid the situation where you have entered, say, 5 percent as the figure 5, then used that value as if it were a fraction. If the 5 percent were dimensionless, no units error would be reported. If, however, you had entered the equation = 5 with units "percent", you would correctly get a units error reported. You would then know to divide this figure of 5 by the 100 ("100percent" constant) to make it 0.05 (the correct fraction). The units would then also be correct.
If you are using fractions, then of course they are all simply dimensionless.
Hope this helps!
Posted: Fri May 02, 2003 2:45 pm
by Salima
Thanks Lee and Tony for your replies.
Actually, I am sorry I didn't ask the question I really intended to.
When I have a percentage or a fraction that is related to time, for example, the rate of increase of the deer population per year is 3% then, the unit (of the rate of increase) is unit/unit/year or 1/year or equally fraction/year.
When the fraction or percentage is not related to time, its unit is dimensionless.
Please correct me if I am wrong.
Thanks. Salima
Posted: Fri May 02, 2003 3:57 pm
by Lee Jones
You are correct.
I was simply trying to help avoid any possibility of problems using percentages as fractions (in your example, if you use the value 0.15 then fine, if you use the value 15, then you need to divide by 100 before using it).