Spreading subscript elements over time

[rubynguyen]

Hello,
I'm working on a simple model to calculate end of life vehicles based on sales. When I used other SD software, I was able to have subscripts as number, and made use of time to spread out the sales by year. However, because Vensim subscripts only accept text, I have to work around by entering a function for each subscript, which is time consuming. Another issue is that I also have to track vehicle age by using a stock, instead of having age as a subscript like the other software.

Is there a better way to model this other than my work around approach? Any feedback is highly appreciated. Thank you!

Ruby

[Administrator]

There is a sample model that ships with Vensim that should help.
On the Mac, you will find it at
/Users/Shared/Vensim/Help/HTML/Models/Sample/EXTRA/TUBS.MDL

on Windows, it's something like
c:\users\public\Vensim\Help\HTML\Models\Sample\EXTRA/TUBS.MDL

This model is what I always start with if I'm looking to capture the ageing process. Tubs is a set of bathtubs that fill sequentially, but it's easy to extend this to an age structure and move everything from one "age" to another after a fixed period of time.

There are also a lot of hits on Google if you search for cohort and Vensim.

[Administrator]

PS. You might want to look at Ventity as well which handles this kind of thing much easier.

[rubynguyen]

Thank you, Admin! I've been thinking about Ventity as well. One general question: once switched to Ventity, is there a reason to come back to Vensim? In other words, is Ventity an improvised version of Vensim? Will Ventana plan on supporting both or eventually will only support Ventity? Thanks!

[tomfid]

Both will be supported for the foreseeable future.

They have different strengths. If you have data with a sparse or ad hoc key structure, or you need dynamic creation/deletion of entities, Ventity makes a lot of sense. But Vensim has some unique capabilities as well, including richer optimization options, web publishing, and simultaneous equation solving.