Archive for June, 2009

Module FAQs

June 29th, 2009 No comments

The version 9.1 release of iThink & STELLA introduced a new approach to modeling complex systems: hierarchical modules.  Modeling with modules can really simplify the way you think about, construct and communicate systems.

Since a lot of customers are now getting their feet wet with modules, we’ve been receiving a steady stream of questions.  We asked our Director of Product Development Karim Chichakly to compile a list of frequently asked questions that he thought would be useful for anyone using modules.  The questions and answers are listed below. If you have any additional questions, post them in the comments and we’ll respond.

What is a module?

CropperCapture[12] A module is a container for lower-level model structure.  It is designed to support a process of top-down model design and development.

What is a module input?

CropperCapture[18]A module input is a variable that can become the ghost of another variable at a different level of the module hierarchy.   It becomes the ghost when you assign it to that other variable.  After it is assigned, it is referred to both as a cross-level ghost and an assigned module input.  All of this is managed transparently if you use the ghost tool.

What is a module output?

CropperCapture[19] When a variable is designated as a module output, you are telling the software that this variable can be ghosted on a different level of the module hierarchy.  While any variable can be a module output, it is important to carefully manage which variables you intend to be shared across modules.

How are modules different from Decision Process Diamonds (DPDs)?

DPDs were designed to collapse one level of structure and hide non-essential details while modules were designed for top-down development of complex models.  Their goals are very different and so is their behavior.  While it is true that modules can be used to replace DPDs, the reverse is not true.

Read more…

What’s New in isee NetSim 1.0.2?

June 16th, 2009 No comments

isee netsim icon

Along with the iThink/STELLA 9.1.3 release, we are happy to announce the isee NetSim 1.0.2 update.  We have fixed many issues and also added a few features:

  • Expanded Support for Navigation Buttons — Navigation buttons now support options to “Restore All Devices”, “Restore Graphs/Tables”, and “Switch Variable On or Off”.
  • Print Interface — Support for the “Print” menu button has been added so that users can now print the current NetSim interface page.
  • Edit Text Boxes — Text boxes that do not have the “Lock Text” option enabled are now editable so that you can provide an input form for users. Coupled with the “Print” menu button, students can now print out their responses to online assignments.
  • Proxy Server Support — Users with an Internet proxy server connection can now configure their isee NetSim Wizard installation to publish models on the web.
  • Trial Version — We are excited to offer a 30-day trial version of isee NetSim.  If you currently don’t own isee NetSim, click here to download the trial.

If you are a current isee NetSim owner, you can login and download the update now on your My Software page.  If you want all the nitty-gritty details, take a look at the complete release notes here.

Categories: isee NetSim Tags: , ,

Version 9.1.3 Updates Key Features

June 16th, 2009 1 comment

We just released another update to STELLA and iThink — Version 9.1.3.  This particular release has a number of updates specific to key features.  For example, if you’re a Macintosh user and rely on the data import/export functionality, you’ll definitely appreciate the updates we made to make sure the software is compatible with the latest versions of Excel for Mac.

Updates to the ARRAYRANK builtin include an optional parameter to specify a secondary sort field for variables with the same value.  You can now also use the ARRAYRANK builtin  in a non-arrayed variable.

My personal favorite in this release, though, is an enhancement we made to the Spatial Map utility. In addition to color configurations, you can now assign an image to a range of values.  This really expands your ability to create  interesting visual representations of spatial data.

forest and modelUsing images to visualize simulations

This simple model of trees burning in a forest illustrates how images can be used in a spatial map configuration.

The model is set up so that initially there is one tree in the middle of the forest that is burning.  All of the other trees in the forest are healthy living trees.

When you run the simulation, the images in Spatial Map allow you to see the fire spreading to adjacent trees. It really adds to the visual effect of what’s happening in the model. Eventually the burning trees die out and you’re left with a forest mostly full of dead trees — now that’s a visual!

If you want to learn more about Spatial Map, check out Karim Chichakly’s series on spatial modeling in iThink and STELLA .

A full list of the features and fixes in Version 9.1.3 is available on our web site.  If your Technical Support Contract is current, you can go ahead and download the update now from your My Software page.

Hope you have as much fun with the spatial mapping as I did!

Building a Health Care Model Hierarchically

June 10th, 2009 2 comments

I recently had the pleasure of building a very large model of the health care system from many small discrete parts.  I did this in a course on Health Care Dynamics taught by James Thompson at Worcester Polytechnic Institute.  The design of the model is entirely Jim’s.

The most striking thing about this model to me is that it was created completely hierarchically.  I have seen many large models broken into sectors which are conceptually all at the same level.  I have seen other large models that are organized by feedback loops, which can at some times be large and unwieldy.  But I had yet to see an example that is truly hierarchical, with an appropriate dynamic hypothesis at each level.

The model is three-levels deep.  At the lowest level are models with very simple dynamic hypotheses.  At the next level up, groups of these smaller models are tied together to form more complex feedback loops, or loop sets, comprising a higher-level dynamic hypothesis (this is the complexity most of the models I develop have).  At the top-level, they are tied together to form a very high-level dynamic hypothesis.  One of the very nice things about this is that each part of the model, which was built bottom-up, has already been tested in isolation or within its group before the whole model is tied together.  All parts are in steady-state.  In this way, we have built confidence in all of the parts of the model and now are only testing the broadest feedbacks.


Critics of this approach insist that delaying the connection of the broadest feedbacks until this late in the development of the model hides important dynamics that affect all parts.  Not only is this considered risky, but the model does not generate results until the end.  After this experience, I can’t agree with this point-of-view.  There was very little risk in tying the pieces together at the end because they were well-formed pieces already rich in feedback, and (very important!) initialized in steady-state.  While the model did not address some of the overarching issues until the end, careful testing of the pieces of the model added insight at many steps along the way and even gave hints about what might happen when those final feedbacks were put into place.

Read more…

Tips, Tricks and Shortcuts for using iThink and STELLA

June 8th, 2009 3 comments

I recently saw an iThink/STELLA power user change the direction of a Flow with a simple keystroke. And I thought the only way to change a Flow direction was to delete and redraw it! This prompted me to search for other tricks about using the software that I might not be aware of. As it turns out there are quite a few! I took a stab at creating a list.

The list ranges from Paintbrush tips to keyboard shortcuts. If there are other tricks that you know about, please post a comment and share them with the rest of us!

On bended flow

Bending a flow


•  Bend a Flow — Drop a Flow and start dragging it. When you want it to change direction at a 90 degree angle, hold the shift key while you move the cursor in the direction you want the Flow to go.

• Change the Flow direction — Hold the Control key and click on the head of the arrow.

• Disconnect a Flow — Click and hold on the end of the Flow (either the arrow end or the tail end) to drag it from a Stock.

Read more…

We are on Facebook

June 6th, 2009 1 comment

FacebookFacebook is all the rage these days, so we have setup a few Pages so you can access this blog and create some conversations there as well.  So, if you use Facebook and want to become a fan, head over to the Pages!

(If Twitter is more of your thing you can follow me at