help to understand how Ventity works

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LAUJJL
Senior Member
Posts: 1421
Joined: Fri May 23, 2003 10:09 am
Vensim version: DSS

help to understand how Ventity works

Post by LAUJJL »

Hi

When one studies Ventity, one feels a bit lost, as the structure of the tool is very different than Vensim's one.

I think that one way to ease that understanding is to see the difference between the Vensim model that addresses the same problem.

I think then that a good idea may be to build right at the beginning and all along the exercises, tutorial or guides, the equivalent model in Vensim, so as to see the differences between the two software and to ease the understanding because one is more accustomed to see the structure of a problem when written in Vensim than in Ventity; of course as long as it is possible.

Regards.

JJ
tomfid
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Re: help to understand how Ventity works

Post by tomfid »

Good idea. We'll see what we can cook up. It may be tricky in some situations, because there are things Ventity does that will be hard to represent in Vensim.
moudi
Senior Member
Posts: 69
Joined: Thu Nov 18, 2010 9:18 am
Vensim version: DSS

Re: help to understand how Ventity works

Post by moudi »

Along the same lines I would wish to have a number of small models or building blocks, that demonstrate how Ventity works.
For example:
- aging chain with a small number of stages; cohort model (not in combination with the car fleet model; only the aging mechanism as such)
- aging chain with leakage
- etc.

It would also be interesting just to know where Ventity works similarly to Vensim. I assume for example, that a SIR-model works the same in principle, except for the fact that a number of different groups could be considered easily in Ventity (e.g. for gender, age, etc. with different infectivities, etc.).

Thanks, Justus
tomfid
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Re: help to understand how Ventity works

Post by tomfid »

I'm working on this.

In a sense, there isn't an "aging" mechanism in a Ventity cohort model. You don't have to move people from one cohort to another. You keep track of a list of cohorts, add new cohorts, and delete old ones if they're (nearly) empty.

Anything that's inside a single entity is essentially identical to Vensim. So, you could build an entity containing an SIR model, and get the same result. Then you could create additional instances representing different populations, with different parameters, which would be much like adding an array dimension to the Vensim model.

You'd start to run into differences if you had [Gender,Occupation] with M,F and Nurse,Pilot. In Vensim that would imply 4 combinations, but in Ventity you could populate just Male,Nurse and Female,Pilot if desired. Of course in Vensim you could have a 1D vector capturing this, with population : maleNurse,femalePilot.

The Ventity difference would really start to show up in 2 situations:
1. If you have a lot of interacting populations, there's a [FromPopulation, ToPopulation] relationship that is often sparse (i.e. only a few of the possible interactions exist). In Vensim, you'd have to define the full matrix with a lot of 0s (though there are some tricks to get around that). In Ventity, you'd simply create a relationship entity and define only the instances that exist.
2. If new diseases emerge, you could create population entities dynamically, as needed, again avoiding the need to run a lot of 0s until array elements are activated.

We have some models that illustrate this. If they aren't in the Help system, I'll have to see that they're added.
moudi
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Vensim version: DSS

Re: help to understand how Ventity works

Post by moudi »

Hi Tom, thanks for your reply

Yes, it would be great to have an example of a population model with a number of age classes, to understand how Ventity works in this regard. Do I understand correctly, that a cohort (say: born between 1961 and 1980) keeps this information as an attribute throughout the simulation with, let's say, a mortality rate depending on their age?

Justus
tomfid
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Re: help to understand how Ventity works

Post by tomfid »

That would be one way to design it, yes. You could also create a conventional aging chain, but I think it's more work without any corresponding improvement in insight.

I'm posting a video and some sample models shortly.
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