what does time step mean

sally · Post by **sally** » Fri Nov 10, 2006 9:45 am

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

LAUJJL · Post by **LAUJJL** » Fri Nov 10, 2006 11:26 am

joined the model 2 that is right
JJ

sally · Post by **sally** » Fri Nov 10, 2006 11:51 am

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 ..

LAUJJL · Post by **LAUJJL** » Fri Nov 10, 2006 2:34 pm

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

bob@vensim.com · Post by **bob@vensim.com** » Sat Nov 11, 2006 12:18 pm

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.

sally · Post by **sally** » Sat Nov 11, 2006 12:32 pm

thank you for your help

i will work through sterman's book ,

hope i can learn more...

thank you