Ventana software support forum

when do i need to use time step in my model?

when i set up a model, i always fact the unit problems,

such as, if invetory is a level (case), customer demand is a Aux(case), and sales(case/day) is Min(inventory, customer demand), i will see the unit wrong message, How to figure it out?

attachment is my practice model.

hope someone can give me suggestion...

thank you

joined the model 2 that is right
JJ

hi,

thank you for your help. but i am confused about " sales rate=MIN(inventory/delivery time, customer demand).

what's the difference between above and "sales rate=MIN(inventory, customer demand)/delivery time....

thank you ..

Another more appropriate model, deals with the time step and not the time delivery is the model 3 joined.
You should study the Vensim\models\guide\chapter3, where you find inventory models.
You can find too plenty of inventory in other models too.
Regards.
JJ

Hi Sally,

sales rate=MIN(inventory, customer demand)/delivery time

is dimensionally inconsistent - demand is widget/month so that demand/deliverytime would be widgets/month/month which makes no sense. inventory/deliverytime converts the stock inventory to a flow. If you can work through a few chapters of a text book on system dynamics it will likely be very helpful.

thank you for your help


i will work through sterman's book ,

hope i can learn more...


thank you