Ventana software support forum

This is probably the most easiest question but I am having serious issue in calculating the demand in my model. I have some stock 'clothes in wardrobe'. Each year 50% goes to landfill and rest is re-used. While clothes stay in wardrobe for almost 7 years. On the basis of stocks I want vensim to calculate the new demand needed to produce every year. How can i model it in vensim. The way I am doing is using a delay function where my constant value of items needed (5 kg of clothes per year per capita) is minus with stock of last year. But I am getting demand for production in negative numbers. This can not be possible because demand of clothes can not go negative as people always buy clothes.
I hope someone can really help me out.

It's hard to say what's going wrong without an example model.

However, one possibility is that what you're suggesting is not possible. If clothes last 7 years, then at most 1/7th can go to the landfill per year.

In any case, your decision rule for buying new clothes must:
- replace clothes discarded
- adjust the stock of clothes toward a goal

Often this implies something like:

clothes ordering = expected clothes discarding + (desired clothes - clothes)/time to adjust clothing stock
expected clothes discarding = SMOOTH( actual clothes discarding, time to perceive clothing discards )