Making Connections

Jeremy Merritt STELLA & iThink

It seems like everyone has been talking about H1N1 (swine flu) the last couple of months.  If you have children in school, then you are probably very aware of how fast the virus is spreading.  Schools are the perfect environment for a virus to spread.  To help understand why, we created a STELLA model of a high school that introduces the H1N1 virus.  You can experiment with vaccination and “stay at home” policies to limit the spread of the flu.

The STELLA model is based on the SEIR compartmental model that epidemiologists use to model the progress of an epidemic.  SEIR models divide the population into compartments: Susceptible, Exposed, Infected and Recovered.  These ‘compartments’ translate nicely into stocks within the STELLA model where we can observe the dynamics of the spreading virus.

While developing the model we decided to explore some strategies that schools are pursuing to limit the virus’ spread.  We wanted to know if the “stay at home” (when you are sick) policy would be effective in the case where vaccines are not available quickly enough, (which as of November 2009 is the case).

Take a look:

Click the ‘Simulate’ link on the home screen above and try some different scenarios.  Be sure to click the ‘How does this simulation work?’ link for a guided tour of the model behind the simulation.

As you experiment with the simulation, consider the following:

• How does varying “% vaccinated” effect the number of sick students?
• How many days do infected students need to stay home to have a significant impact on the spread of the virus within the school?
• What impact does the “% effectiveness of vaccine” have on the flu outbreak?
• What combination of decisions results in the lowest number of sick students?  Are these decisions realistic in a real-world setting?

Note: Each time you dial in parameters and press run, a new plot will be added to the graph so you can compare the effectiveness of the different decisions.  Clicking on the blue reset button will clear the graph and reset all Knobs to their default value.

If you think this simple model is useful, feel free to share it or embed it on your own website; just click the sharing icon in the lower right corner.  If you want to dig deeper into the STELLA model you can download the model by clicking here.  You can open the model with STELLA 9.1, or the free isee Player.

embedded, epidemic, flu, h1n1, infection, netsim, seir

Jeremy Merritt News & Announcements

Last Friday we hosted the first of a four-part web seminar series titled Modeling for Environmental Sustainability.  This first session was about how you can use simple bathtub dynamics to understand climate change.

In the session, Chris Soderquist demonstrates how you can use embeddable simulations to help raise awareness and understanding about climate change.  Using simple bathtub dynamics, the “C02 Dynamic Indicator” simulation lets anyone experiment with the current set of policy proposals for capping and reducing emissions.

The live session was recorded and is now posted on our website, along with two embedded simulations that you can run online.  All of the session materials are open source and we encourage you to link, embed and share them with others.

c02, Climate Change, netsim, policy, webinar

Jeremy Merritt Modeling Tips

A few months ago, we designed an interface template for our isee NetSim sample models.  The template is an iThink/STELLA model in which the interface layer is designed to look like a website:

• Navigation buttons are styled to look like hyperlinks instead of push buttons.
• Push buttons are used to perform actions, such as running the simulation.
• Tabs along the top of each page let the viewer know which section they are in, as well as providing a way to quickly jump to any section.

An example of the interface template applied to a model published online

You can download the template model and use it as a starting point for creating interfaces to models you want to publish online.

Download the template for STELLA

Download the template for iThink

How to use the template

The template model consists of four interface pages: Home, Background, Explore Model and Simulate.  Navigation buttons to and from each page are already setup within the interface.  You can customize each section by replacing the placeholder text.  The pages are a starting point that establishes a website-like look and feel.

The easiest way to apply the template model’s interface to your own model is to simply copy and paste the pages.

1. Open your own model and switch to the Interface layer.  Choose ‘Interface Prefs’ from the Interface menu and make sure the ‘Show Pages’ option is selected.  Set Page Size to 800×600.  This will allow you to see the page boundaries as you apply the template.
2. Open the template model and from the Interface layer choose the Edit menu –> Select All, then Edit menu –> Copy to copy all of the pages to the clipboard.
3. Go back to the interface layer of your model and choose Edit menu –> Paste.

When you have pasted the pages into your model, they will probably not be lined up correctly within the page boundaries.  The dashed lines show the boundaries of each page.  Using these dashed lines as a guide, use the arrow keys on your keyboard to make the pasted pages line up properly.

Now you can customize the title and text in each page.  Add a picture to the home page and build a control panel.  If you want, you can use the hyperlink button on the ‘Background’ page to link to online information, (PDFs, other models, Wikipedia, etc.).  Consider building a storytelling sequence on the ‘Explore Model’ page to unfurl the model a chunk at a time.  These are just a few customizations you can make.  Take a look at some of our sample models published online with isee NetSim for ideas.

If you publish some models online that use the template, we’d love to see them!  Leave a link in the comments so we can check them out!

interface, netsim, template

Jeremy Merritt Stories from the Field

There is no doubt that the Car Allowance Rebate System, or “Cash for Clunkers” program has proved to be popular with American car buyers.  So popular in fact, that it ran out of cash within the first six days of processing claims.  This news got me thinking about the program in a more systemic way.

Whether you think of the program as a stimulus for the economy or the environment, it cannot continue to operate without funds.  Congress had to quickly approve another $2 billion for the program when the initial $1 billion appropriation was exhausted in less than a week.  The rebate fund lacks an inflow of cash to offset the outflow of payments.

I wrote about a STELLA model of a self-financing program to promote cleaner vehicle sales in May.  The “Feebates” model uses fees charged on sales of vehicles that have high pollution rates to finance rebates for cleaner vehicles.  I asked the Feebates model author, Andrew Ford what he thought about “Cash for Clunkers”.  He sent me an article he co-authored with Todd BenDor titled “Simulating a combination of feebates and scrappage incentives to reduce automobile emissions”.  Basically, the article describes a feebates model with a “cash for clunkers” type of rebate added to promote scrapping older vehicles.  The article was published in the journal Energy, 31(2006), 1197-1214.

Mr. BenDor was kind enough to send me the STELLA model so we could publish it to the web with isee Netsim.  We thought this would be a great way to start some discussion around the Clunkers program, public policy and using simulations to facilitate decision making.

Click here to run the simulation online.

• Can you design a policy that stimulates the economy and reduces emissions?
• This simulation uses fees to fund the program instead of deficit spending.  What other sources of revenue could we consider?
• Many states levy a “sin tax” on tobacco and alcohol.  How would the public react to fees on high-pollution vehicles?

Post your answers in the comment section below.

Image credit: http://www.flickr.com/photos/threadedthoughts/3808715790/

"cash for clunkers", feebates, netsim, policy

Jeremy Merritt Modeling Tips

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?

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?

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?

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…

Decision Process Diamonds, iThink/STELLA, modules

Jeremy Merritt isee NetSim

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.

proxy, trial, updates

Jeremy Merritt News & Announcements

Facebook 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!

• isee systems Facebook Page
• STELLA Facebook Page
• iThink Facebook Page

(If Twitter is more of your thing you can follow me at http://twitter.com/jeremyx)

facebook, twitter

Jeremy Merritt Education

Imagine you have been asked to design an incentive program to promote the sale of cleaner vehicles in your city. You are convinced that purchase price incentives are the key to shifting consumer choices. You believe a large rebate could encourage the purchase of electric cars and other alternative-fueled vehicles.

This is the background story for an online simulation that allows students to experiment with applying fees and rebates to automotive sales to help reduce air pollution.

As the player, your job is to manage a “feebate” program for the city that will help reduce vehicle pollution.  The program offers large rebates on purchases of cleaner vehicles, (electric vehicles, hybrid-electric vehicles and vehicles fueled with compressed natural gas) and imposes fees on vehicles that have high pollution rates.

Read more…

emissions, environment, game, netsim, policy