Beginner - First Model

[tmonteit]

All,

I'm teaching myself Vensim and working through my first model. I've read a few chapters and studied many of the examples. You don't really learn new tools until you push off on your own. I've hit a couple of stumbling blocks with the tool mechanics and I hope some experts on the forum could add insight and throw your spin onto this simple model.

I thought I would start with something pretty elementary a Lemonade Stand Model (inspired by my entrepreneurial neighbor kids).

Model Scenario:
1. I set this model up to calculate daily and run for 2 years.
2. The kids have up front startup costs: Building Materials, supplies, etc.
3. Revenue comes from the sale of drinks. Drinks start off slow and grow over time (word of mouth, repeat customers, etc.)
4. I want to model the business growth of this scenario.

Issues:

My problems are mostly the vensim tool mechanics.
1. How do I make the DRInks sold to show functional growth? I would like drinks sold to S-curve input function from 10 to 120.

2. How do I setup Units on the flow rates? - Drinks are sold on a daily basis, but the kids have monthly expenses. Vensim seems to be ignoring this units. Shouldn't the tool convert my flow rates automatically?

Also, for the modeling experts: Have I correctly modeled this scenario? The attached file shows the business 'loosing the farm' but this shouldn't be the case.

What variations could I add to teach me more about modeling and make this an educational scenario?

Thanks for advice...

T

[tmonteit]

Also, forgot to mention that the kids only work weekends. How would I incorporate that aspect into the model?

[rdudley]

I suggest that you look at the system dynamics roadmaps site at http://sysdyn.clexchange.org/road-maps/rm-toc.html especially part 8 (I think) which has a lemonade model ..... there are many similar models about which can provide guidance.

If you are interested in education scenarios look at information here: http://lexchange.org/

Also, are you teaching yourself Vensim, System Dynamics Modeling, or both. Learning the software is only a small part of the task as I think you realize

[LAUJJL]

Hi

Joined a modified model.
Not a profitable business!
Regards.
JJ

[tmonteit]

I am teaching myself both systems modeling and vensim. Thank you both for the input. Your links and model were very helpful. I had not seen the clexchange.org site.