Search found 55 matches

by Tom Fiddaman
Wed Jan 05, 2005 2:23 am
Forum: 2004
Topic: Models and Science
Replies: 20
Views: 24129

Models and Science

Posted by Tom Fiddaman <tom@vensim.com> ""The environmentalists"" are as handy a punching bag as ""economists."" Yet the image of wild-eyed greenies running around predicting the apocalypse and monkeywrenching the economy has little to do with the mainstream of environmentalism, the substance of the...
by Tom Fiddaman
Tue Dec 21, 2004 9:07 pm
Forum: 2004
Topic: Models and Science
Replies: 20
Views: 24129

Models and Science

At 12:29 AM 12/17/2004, Mark Wallace wrote: >They still can't tell me if it's going to rain tomorrow. The charts cited >above rely on something called a ""Heidke Skill Score,"" which is a >controversial verification technique, even within NOAA: >http://ams.confex.com/ams/84Annual/techprogram/paper_6...
by Tom Fiddaman
Wed Dec 15, 2004 7:48 am
Forum: 2004
Topic: Models and Science
Replies: 20
Views: 24129

Models and Science

At 09:30 AM 12/9/2004, Mark Wallace wrote: > > Re: ""Modeling is not science..."" > > > > Sir Isaac and his later colleague Albert would disagree. >I wouldn't be too sure about that. As the original poster pointed out, >this quote was taken out of context (to say the least). >My belief is that Mr. E...
by Tom Fiddaman
Mon Dec 06, 2004 7:43 pm
Forum: 2004
Topic: Models and Science
Replies: 20
Views: 24129

Models and Science

Someone should take away Ebell's cell phone and any other modern convenience created with the assistance of a model. >While all models have their limits, and many are fatally flawed, there is >actually a mental model in action to dismiss formal models. In Ebell's case >it is the inconsistency with h...
by Tom Fiddaman
Mon Nov 22, 2004 4:38 pm
Forum: 2004
Topic: The Derivative in the Real World
Replies: 9
Views: 11981

The Derivative in the Real World

In the interest of stirring the pot, I'll make a few bold (but not really original) assertions about derivatives: Generally I find it pointless (though admittedly fun) to debate whether differential or integral equations are a better mental model or description of reality given that the models we're...
by Tom Fiddaman
Mon Nov 01, 2004 9:22 pm
Forum: 2004
Topic: The Derivative in the Real World
Replies: 9
Views: 11981

The Derivative in the Real World

>I still say that mountain sides (i.e. slopes) are examples of derivatives >with respect to distance in the real world. I think this suffers from the same problem as the idea of time derivatives in Forester's argument: how would you measure the slope of a mountainside, except by integrating (i.e. me...
by Tom Fiddaman
Mon Aug 04, 2003 4:43 pm
Forum: 2003
Topic: Global warming and structure versus behaviour
Replies: 2
Views: 5721

Global warming and structure versus behaviour

I dont have the book cited by Geoff Coyle, but Ive put corresponding data from the Vostok ice core and other sources on a web page: http://www.sd3.info/CO2temp.html I included other time scales (420,000 years, ~200 years) so you can draw your own conclusions. I have to agree with Sterman & Moxnes th...
by Tom Fiddaman
Fri Feb 21, 2003 12:24 pm
Forum: 2003
Topic: attractivenss multiplier
Replies: 6
Views: 8331

attractivenss multiplier

If you surf the web for "random utility maximization" "discrete choice" "LOGIT" "generalized extreme value" "MCI market share" and similar terms youll find a lot of references to this. Ill save you the trouble by listing three comprehensive online books: http://elsa.berkeley.edu/books/choice2.html h...
by Tom Fiddaman
Wed Nov 20, 2002 10:35 am
Forum: 2002
Topic: Ecological Footprint of Humanity
Replies: 10
Views: 7386

Ecological Footprint of Humanity

Im generally open to the idea that humans are intelligent, but otherwise Im a little more pessimistic than George Backus: At 08:56 AM 11/17/2002 -0700, you wrote: >In responding to Bill Harris: Lomborg argues that most of the reports >excess "ecological footprint" is energy. The energy intensity is ...
by Tom Fiddaman
Wed Jul 24, 2002 1:25 pm
Forum: Up to 1998
Topic: Differential equations
Replies: 25
Views: 23663

Differential equations

>Derivatives are found (in abundance) in nature. And thus provide >derivative meters. For example, an (electrical) inductor has the >relationship that a change in current flow through it creates a voltage >across it. Change the speed of an electron in motion and it radiates >electromagnetic energy i...
by Tom Fiddaman
Wed Jul 03, 2002 2:29 pm
Forum: 2002
Topic: Representing constraints
Replies: 30
Views: 27829

Representing constraints

I like Alan Grahams approach to gas laws as it gets at an operational description of the problem, which will probably be more intuitive/interesting to students than just playing with the algebra. But assuming that you really do want to play with the algebra, the obvious output is a 3d surface plot -...
by Tom Fiddaman
Thu Jun 20, 2002 11:21 am
Forum: 2002
Topic: SD Country model
Replies: 10
Views: 6686

SD Country model

>>There are a number of doctoral dissertations on various sectors or other >>aspects of the National Model or modeling an economy. Among >>dissertations you might be interested in ... Peter Senge (government >>sector), Peter Senge did the investment function. Barry Richmond worked on government, tho...
by Tom Fiddaman
Tue Apr 16, 2002 8:46 pm
Forum: 2002
Topic: Modeling Anticipation
Replies: 8
Views: 8635

Modeling Anticipation

Bill - Im going to take Jay Forresters advice and speak in black and white for those listening in shades of gray, so take this note with a grain of salt. A frequent approach to forecasting in models is to assume that they are perfect, by using optimization to make the decision in question. Many econ...
by Tom Fiddaman
Tue Dec 11, 2001 4:14 pm
Forum: 2001
Topic: Text Based SD software
Replies: 15
Views: 12792

Text Based SD software

At 09:58 AM 12/6/2001 -0500, you wrote: >Lazaros Petrides asks for text-based SD software that does "an automatic >translation of flow diagrams into dynamo code and vice-versa." The real obstacle to literal translation from flow diagrams into equations is that a causal loop or stock-flow diagram con...
by Tom Fiddaman
Fri Dec 07, 2001 2:42 pm
Forum: 2001
Topic: Text Based SD software
Replies: 15
Views: 12792

Text Based SD software

One of the problems with translators is that writing them requires understanding the language on each end. Because there isnt a lot of call for model reuse (another topic worth discussing) there isnt much commercial incentive for developers to get together or learn one anothers nitty gritty details....
by Tom Fiddaman
Thu Dec 06, 2001 10:44 am
Forum: 2001
Topic: A causes B
Replies: 14
Views: 17776

A causes B

One other notion of causality you may bump into is "Granger Causality". The essential idea is to ask yourself, "if I knew about everything in the universe except x, could I still predict y?" If x doesnt help predict y, one concludes that x does not Granger-cause y. I sometimes find this a useful tho...
by Tom Fiddaman
Tue Oct 16, 2001 9:50 am
Forum: 2001
Topic: Is it possible to omit soft variables?
Replies: 8
Views: 9811

Is it possible to omit soft variables?

Im with Jay on this one. Omission of unmeasured variables is not a helpful strategy for most purposes. Including soft variables in models may subject us to some uncertainty, but if we exclude them we cant even talk about 90% of the interesting problems in the world. The technical distinctions betwee...
by Tom Fiddaman
Sun Aug 12, 2001 8:29 pm
Forum: 2001
Topic: Preventing the next boom and bust
Replies: 12
Views: 10587

preventing the next boom and bust

Martin Taylors characterization of the reinforcing process driving speculation is accurate. However, I think you will find few economists who would agree that "Purely rational behaviour on the part of the investors, with full information (other than the instantaneous plans of other investors) would ...
by Tom Fiddaman
Wed Jan 10, 2001 7:47 pm
Forum: 2000
Topic: Building your own models
Replies: 18
Views: 49123

Building your own models

At 08:10 AM 12/1/2000 +0000, Kim Warren wrote: >So may be we shouldnt be at all afraid to embrace statistical methods for >understanding the directly causal relationships that do exist, so long as we >dont pursue them across stock boundaries. Its safe and useful to cross the stock boundary as long a...
by Tom Fiddaman
Tue Jan 02, 2001 10:28 am
Forum: 2000
Topic: Diabetics SD models
Replies: 3
Views: 8827

Diabetics SD models

You might take a look at: Jeppe Sturis, Kenneth S. Polonsky, Erik Mosekilde, and Eve van Cauter. Computer Model for Mechanisms Underlying Ultradian Oscillations of Insulin and Glucose. Am. J. Physiol. 260 (Endocrinol. Metab. 23): E801-E809, 1991 The model is in the model library on my web site ( htt...
by Tom Fiddaman
Tue Nov 14, 2000 2:53 pm
Forum: 2000
Topic: Delays in SD modeling!!
Replies: 15
Views: 47223

Delays in SD modeling!!

At 10:28 AM 11/12/2000 +0000, Geoff Coyle wrote: >Bob Eberlein correctly reminds us that the common practice of using DELAY3 >may mislead and we should be more careful. I dont think that he is right in >saying that Vensims pipeline delay has an infinite number of hidden levels. >It is more likely to...
by Tom Fiddaman
Mon Oct 30, 2000 10:17 am
Forum: 2000
Topic: Phd Theses online?
Replies: 2
Views: 8378

Phd Theses online?

>[Hosts note: Many MIT theses are available at http://theses.mit.edu ] The trick at the MIT theses site is finding things. Searching for terms like "system" and "dynamics" is unhelpful. You might try some of the following authors: - Bent Bakken - Ernst Diehl - Rogelio Oliva - Anjali Sastry - David F...
by Tom Fiddaman
Fri Jul 28, 2000 9:36 am
Forum: 2000
Topic: Tracking purchase prices on past transactions
Replies: 6
Views: 6579

Tracking purchase prices on past transactions

Bill - At the risk of sounding like a broken record, what problem are we trying to solve with the detailed tracking? Vensim 4.1 includes new QUEUE functions that solve half your problem - tracking attributes as a coflow with the queue elements. Unfortunately, theyre also FIFO, so unless you move the...
by Tom Fiddaman
Mon Jun 26, 2000 9:32 am
Forum: 2000
Topic: SD Humour
Replies: 10
Views: 6417

SD Humour

>How many system dynamicists does it take to screw in a lightbulb? > Wait. Lets back up and ask why we want the lightbulb screwed in. What problem are we trying to solve? Or: None. Theyd rather sit in the dark and argue about whether systems thinking is a subset of system dynamics, or vice versa. Or...
by Tom Fiddaman
Sun Jun 18, 2000 11:01 am
Forum: 2000
Topic: linking flow to flow directly?
Replies: 9
Views: 31425

linking flow to flow directly?

> "Try linking the inflow > directly to the outflow > (bypassing the stock) ... > Perhaps Im missing something here, but > isnt it a violation of the fundamental > concepts of the System Dynamics approach > to do this? This idea is so important that Forrester mentions it three times in Industrial Dy...