units for percentages in Vensim models

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Salima
Junior Member
Posts: 10
Joined: Sat Apr 26, 2003 10:08 am

units for percentages in Vensim models

Post 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
Administrator
Super Administrator
Posts: 4573
Joined: Wed Mar 05, 2003 3:10 am

Post 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.
Lee Jones

Post 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!
Salima
Junior Member
Posts: 10
Joined: Sat Apr 26, 2003 10:08 am

Post 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
Lee Jones

Post 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).
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