adjusted time

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swapple
Junior Member
Posts: 8
Joined: Tue Oct 27, 2020 6:35 am
Vensim version: PLE

adjusted time

Post by swapple »

Hello,

in most of the models built in SD and Vensim, there is a time to adjustment. For example time needed to reach the any target.
there is a question here, what exactly thins time mean?
If we gave a target inventory equals to 100, and the current inventory is 10, and AT (Adjusted Time) is 3 months,what does this 3 mean?
it doesn't mean that it takes 3 months to get to the target. Am I right? because after 3 months it reaches to about 70. So what does this mean exactly? and in real world how should we determine its value?

Thanks
tomfid
Administrator
Posts: 3806
Joined: Wed May 24, 2006 4:54 am

Re: adjusted time

Post by tomfid »

In a typical linear system, like inventory adjustment, this is the time constant of the adjustment process. https://en.wikipedia.org/wiki/Time_constant In fact, the mean adjustment does occur at 3 months, but in a first order system, the distribution is broad, with the largest adjustment happening immediately, and a long tail to infinite time. The rule of thumb is that 95% of the adjustment will be done at three time constants (i.e. month 9).

Ideally, you'd get the value by calibrating to data. Failing that, you can ask someone or reason from first principles (for example, using the 95% rule). Or, just do some sensitivity analsysis to establish a value. Fortunately it often just doesn't make much difference, unless inventory itself is the focus of the model.
swapple
Junior Member
Posts: 8
Joined: Tue Oct 27, 2020 6:35 am
Vensim version: PLE

Re: adjusted time

Post by swapple »

thanks
you mean if sb says it takes 3 month to build the houses or get the goods, we should not enter 3? Am I right?
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