how to convert EXP dynamo to vensim

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alvinpoon
Member
Posts: 32
Joined: Sun Mar 01, 2020 6:23 am
Vensim version: PRO

how to convert EXP dynamo to vensim

Post by alvinpoon »

May I know how to formluate the rate equarion with VENSIM built-in functions for this DYNAMO exponential function EXP ?

e.g. Birth growth at a rate of 3% since year 2015 till now for initial population 1,000 people (GROWTHN=1000)

in DYNAMO , it would be like

GROWTH.KL = GROWTHN * EXP(0.03(TIME.K, 2015)

Thank you.
tomfid
Administrator
Posts: 3804
Joined: Wed May 24, 2006 4:54 am

Re: how to convert EXP dynamo to vensim

Post by tomfid »

It appears that some punctuation is missing or incorrect in that equation.

I would expect the true meaning to be something like:

Growth = GrowthN * EXP( 0.03 * (Time-2015) )

Also, embedding numbers in the equation is generally bad practice. For dimensional consistency and parameter testing, this should be rewritten as:

Growth = GrowthN * EXP( Growth Rate * (Time-Ref Year) ) ~ dmnl
GrowthN = ... ~ dmnl
Growth Rate = .03 ~ fraction/year
Ref Year = 2015 ~ year

"Growth" isn't a great variable name, so it would be nice to replace it with something more descriptive.
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