Comments on: Integration Methods and DT

By: David

David — Tue, 18 Jan 2011 20:34:00 +0000

Thank you Richard, I did. The error was related with usage of PULSE function within IF-THEN-ELSE statement. Now the model works fine, regardless of DT value.

By: Richard Turnock

Richard Turnock — Tue, 18 Jan 2011 18:50:00 +0000

DT does not have any units. DT is not time. Modelers should not be using IF-THEN-ELSE or any other modeling trick to compensate for using the wrong DT. There are ways to tell if the DT is too big or too small. Find someone who knows how to explain DT properly.

By: kchichakly

kchichakly — Sat, 31 Jul 2010 02:54:34 +0000

Hi David, Unfortunately, there are ways to make your model DT-dependent. These usually involve discrete formulations, such as using an IF-THEN-ELSE to decide on the value of the flow rather than setting the flow to a continuous value (such as 20). If you are doing this discretely, you need to divide the flow value by DT to compensate for the fact that the flow value is multiplied by DT before being added to (or subtracted from) the stock. If this does not point you in the right direction, please e-mail your model and question to support@iseesystems.com and we will get right back to you.

By: david

david — Fri, 30 Jul 2010 11:29:10 +0000

I am modeling the dependence of production from workforce, for instance adding 20 people to initial 20. The time unit is days. There are 300 workdays. If dt=1, the sum is 40 people. But if I set DT to one-half of the smallest time constant (day), I end up with only 30 people after 300 days, or with 25, if DT=0.25. Does it means that decreasing DT below actual time unit can distort the results?

By: Agung

Agung — Sat, 24 Jul 2010 05:04:32 +0000

thank you Mr Jeremy Merritt, i'll send it with the problem that i dont understand.

By: Jeremy Merritt

Jeremy Merritt — Fri, 23 Jul 2010 18:15:33 +0000

Hi Agung. Send an email (and attach the model) to support@iseesystems.com and we'll see if we can help you figure this out.

By: Agung

Agung — Fri, 23 Jul 2010 03:48:19 +0000

hello, i was using stella 9.13 for my group discussion, and there's something that i dont understand, it's about making normal distributiton,
i thought we have inserted the right comand,, Norm(means,stddev),
but after we run the simulation for some periods, the graph, sometimes, was not looked like a normal distributed datas, how could this happen?
sorry if my question is out of the topic

By: Why DT Matters | Making Connections

Why DT Matters | Making Connections — Mon, 19 Jul 2010 21:13:19 +0000

[...] [...]

By: kchichakly

kchichakly — Fri, 16 Jul 2010 21:03:23 +0000

Thank you for pointing that out! My editors and I missed that typo – and I should have double-checked with a calculator.

By: Akbar

Akbar — Fri, 16 Jul 2010 20:05:27 +0000

In the second paragraph, you have:

2nd year: interest = $1100 × 0.1 = $110; Balance = 1110 + 110 = $1220

This should be 1100 + 110 = $1210, and the following lines accordingly modified.