Use of the TREND and DELAY functions in Vensim
Use of the TREND and DELAY functions in Vensim
I am new to Vensim and I am trying to use the TREND and DELAY functions in Vensim in building a model for workforce movements. I read the definition for each of the functions, but it is still difficult for me to discern what specific information I need to write the equations. Is there a youtube that can take me through the process of how to use my data to write the equations for these functions? Thank you.
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Re: Use of the TREND and DELAY functions in Vensim
I don't think there are videos on how to use these functions.
What part of the documentation are you struggling with?
What part of the documentation are you struggling with?
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http://www.ventanasystems.co.uk/forum/v ... f=2&t=4391
Units are important!
http://www.bbc.co.uk/news/magazine-27509559
Re: Use of the TREND and DELAY functions in Vensim
There's a sample model for each, attached to the function's topic in the Help system.
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Re: Use of the TREND and DELAY functions in Vensim
See also:
Sterman, John D. 1987. Expectation Formation in Behavioral Simulation Models. Behavioral Science, 32:190-211.
Sterman, John D. 2000, Business Dynamics, Boston, MA: McGraw-Hill. Chapter 16.
And:
https://metasd.com/2011/04/delay-sandbox/
Sterman, John D. 1987. Expectation Formation in Behavioral Simulation Models. Behavioral Science, 32:190-211.
Sterman, John D. 2000, Business Dynamics, Boston, MA: McGraw-Hill. Chapter 16.
And:
https://metasd.com/2011/04/delay-sandbox/
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Re: Use of the TREND and DELAY functions in Vensim
I see these examples in the definition of the TREND function, but I don't know how to translate the information I have into the equation. For example, I have a 'workforce level' variable that has historically increased on an average of 6% per year, which translates to .05% per month. How do I incorporate this trend information into the TREND equation provided, if I want to simulate workforce growth over the next 10 years, one month at a time?
Returns the average fractional growth rate (negative for decline) in the input. Equivalent to the equations:
TREND=ZIDZ(input-avval,average time*ABS(avval))
avval=INTEG((input-avval)/average time,input/(1+ini*averate time))
The TREND function provides a very simple trend estimate for a variable. It really only makes sense for numbers that are always positive and don’t get very close to zero. If you want to trend a number that may be positive or negative the TREND function is not very useful. Instead, you should use the fractional trend relative to a quantity that is always positive (for example change in profit as a fraction of gross income).
Units: TREND(units,time,1/time) --> 1/time
Units for the average time must match those of TIME STEP.
Example
R = 10 + RAMP(1,30,70)
TR = TREND(R,5,.08)
Returns the average fractional growth rate (negative for decline) in the input. Equivalent to the equations:
TREND=ZIDZ(input-avval,average time*ABS(avval))
avval=INTEG((input-avval)/average time,input/(1+ini*averate time))
The TREND function provides a very simple trend estimate for a variable. It really only makes sense for numbers that are always positive and don’t get very close to zero. If you want to trend a number that may be positive or negative the TREND function is not very useful. Instead, you should use the fractional trend relative to a quantity that is always positive (for example change in profit as a fraction of gross income).
Units: TREND(units,time,1/time) --> 1/time
Units for the average time must match those of TIME STEP.
Example
R = 10 + RAMP(1,30,70)
TR = TREND(R,5,.08)
Re: Use of the TREND and DELAY functions in Vensim
In that case, you don't want the TREND function at all. You want something like a stock of Workforce, with an inflow of hiring or workforce growth or similar, where the inflow (people/month) equals the stock (people) multiplied by a growth rate (fraction per month, e.g. .005).
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Bookmarks: http://delicious.com/tomfid/SystemDynamics
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