## Search found 2590 matches

- Wed Apr 07, 2010 1:31 am
- Forum: Vensim
- Topic: The question of optimization function
- Replies:
**3** - Views:
**953**

- Tue Apr 06, 2010 10:00 pm
- Forum: Vensim
- Topic: The question of optimization function
- Replies:
**3** - Views:
**953**

A good start is to take a look at the online help; look in the index for "Payoff Computation" In short, for calibration optimization, the payoff is the sum over time of weight*(model-data)^2. For policy optimization, it's just the sum of weight*modelvar. Kalman filter payoffs are slightly more compl...

- Mon Apr 05, 2010 7:03 pm
- Forum: Vensim
- Topic: New user with newbie problems..please help
- Replies:
**17** - Views:
**4036**

- Mon Apr 05, 2010 3:50 pm
- Forum: Vensim
- Topic: Rounding the value of levels.
- Replies:
**3** - Views:
**919**

If you're using Vensim DSS, it's handy to put JJ's formula in a macro, like :MACRO: ROUND(x,digits) ROUND = INTEGER(x*10^digits+0.5)/10^digits ~ x ~ | :END OF MACRO: a test: digits= 1 ~ [-2,2,1] ~ | test x= ROUND(453.123,digits) ~ ~ | Note that precision is also limited by the floating point format.

- Thu Apr 01, 2010 9:53 pm
- Forum: Vensim
- Topic: Rainfall probabilities
- Replies:
**30** - Views:
**4198**

I posted a version of the Pink Noise generator that includes a :MACRO: version here: http://models.metasd.com/2010/03/pink-noise/

Tom

Tom

- Tue Mar 30, 2010 8:20 pm
- Forum: Vensim
- Topic: New user with newbie problems..please help
- Replies:
**17** - Views:
**4036**

I'd strongly urge you to reconsider. The basic stock-flow structure here seems quite well articulated. Revising the rates so that they rely only on information actually available ( e.g., stock/processingTime ) would be a quite simple change here. A look at the sample project model in Vensim>Models>M...

- Tue Mar 30, 2010 1:28 pm
- Forum: Vensim
- Topic: New user with newbie problems..please help
- Replies:
**17** - Views:
**4036**

JJ is correct on the level-rate sequence. If you connect rate A to rate B directly, that's a formal statement that the causal effect of A on B is instantaneous. If you connect rates circularly, A -> B -> C -> A as you've done in the draft model, then A, B, C represent a system of simultaneous equati...

- Tue Mar 30, 2010 12:01 am
- Forum: Vensim
- Topic: New user with newbie problems..please help
- Replies:
**17** - Views:
**4036**

A few other observations: Generally it's easier to think of systems like this in continuous time (the limit as TIME STEP gets small) rather than discrete x(t) = f( x(t-1) ) notation. For most purposes, a test of a good model with an appropriate time step is to simply halve the time step and see if t...

- Mon Mar 29, 2010 11:35 pm
- Forum: Vensim
- Topic: New user with newbie problems..please help
- Replies:
**17** - Views:
**4036**

I took a look at the model. The basic conceptual problem is the rate-to-rate connections from Rework Error Generation Rate to Rework Error Detection Rate to Rework Rate (and back). Making a rate to rate connection amounts to saying that the relationship is instantaneous, which would imply that error...

- Mon Mar 29, 2010 2:17 pm
- Forum: Vensim
- Topic: New user with newbie problems..please help
- Replies:
**17** - Views:
**4036**

One more reference on project models: David Ford's thesis: http://dspace.mit.edu/handle/1721.1/11105

Tom

Tom

- Mon Mar 29, 2010 2:15 pm
- Forum: Vensim
- Topic: New user with newbie problems..please help
- Replies:
**17** - Views:
**4036**

Another good reference is Tarek Abdel-Hamid's dissertation. It's in a book, with Stu Madnick: http://www.amazon.com/Software-Project-Dynamics-Integrated-Approach/dp/0138220409/ref=sr_1_1?ie=UTF8&s=books&qid=1269871961&sr=8-1 (cheap if you buy a used copy) The original thesis is free here: http://hdl...

- Thu Mar 25, 2010 3:03 pm
- Forum: Vensim
- Topic: Subscript ranges
- Replies:
**2** - Views:
**759**

One way to do this is to forget about mapping and use a matrix to capture all possible interactions between age groups. Here's an outline of a simplified version of your problem: age: (age1-age4) fromAge <-> age toAge <-> age totalInfections[toAge] = sum(infections[fromAge!,toAge]) infections[fromAg...

- Thu Mar 25, 2010 2:52 pm
- Forum: Vensim
- Topic: Rainfall probabilities
- Replies:
**30** - Views:
**4198**

Along the lines of JJ's thinking: Discreteness probably matters, in the sense that daily rain of 1mm is different from 30mm once a month. However, it's possible that randomness in rainfall arrival is not important, or hides insight. In that case, you could model the short-term rain process as a PULS...

- Wed Mar 24, 2010 9:34 pm
- Forum: Vensim
- Topic: Rainfall probabilities
- Replies:
**30** - Views:
**4198**

Here's a model that's close to what I had in mind (no SAMPLE IF TRUE though). Rainfall variation is decomposed into two bits: Long term, year-to-year variability (drought, ENSO, or whatever) is a lognormal term applied to the mean seasonal distribution. Short term, day-to-day variation in rainfall i...

- Wed Mar 24, 2010 9:06 pm
- Forum: Vensim
- Topic: Rainfall probabilities
- Replies:
**30** - Views:
**4198**

- Wed Mar 24, 2010 9:04 pm
- Forum: Vensim
- Topic: Rainfall probabilities
- Replies:
**30** - Views:
**4198**

- Wed Mar 24, 2010 4:17 pm
- Forum: Vensim
- Topic: Rainfall probabilities
- Replies:
**30** - Views:
**4198**

Looking at rainfall_testing_4, a few observations: The Sterman pink noise structure isn't as good as Ed Anderson's version. I'll post the latter when I get a chance. It's weird that "effect of drought" goes negative - seems unphysical. The realized rainfall doesn't track the input lookup "rainfall p...

- Tue Mar 23, 2010 2:43 pm
- Forum: Vensim
- Topic: Rainfall probabilities
- Replies:
**30** - Views:
**4198**

Another sampling approach, simpler than pink noise, is: rainfall = rainfall amount*rainfall occurs rainfall occurs = IF THEN ELSE( rainfall random input > rainfall prob, 1, 0) rainfall random input = SAMPLE IF TRUE( PULSE TRAIN(...) > 0, RANDOM UNIFORM( ... ), RANDOM UNIFORM( ... ) ) rainfall amount...

- Sat Mar 13, 2010 1:50 pm
- Forum: Vensim
- Topic: Move items from one view to another
- Replies:
**2** - Views:
**687**

- Wed Mar 10, 2010 12:14 am
- Forum: Vensim
- Topic: Optimize for particular value at a specific time
- Replies:
**15** - Views:
**2119**

Originally posted by karankhosla Thanks Tom! I'm going to try it out. I presume the logic behind it is that, as the weight is multiplied to the payoff at each time step, the pulse function forces the value of the payoff to be "x" at a particular time and zero elsewhere, which forces the optimizer t...

- Wed Mar 10, 2010 12:05 am
- Forum: Vensim
- Topic: using reality checks to build models
- Replies:
**6** - Views:
**1525**

I think the idea of using RCs in a more automated fashion for model specification is interesting. In a way, the usual SD approach to specifying lookups is a bit like RCs. Good practitioners ask a series of questions: what happens at 0? what happens at high values? ... Those get at SME knowledge abou...

- Fri Mar 05, 2010 3:33 pm
- Forum: Vensim
- Topic: Extract data from subscripts
- Replies:
**4** - Views:
**945**

- Tue Mar 02, 2010 10:51 pm
- Forum: Vensim
- Topic: Optimize for particular value at a specific time
- Replies:
**15** - Views:
**2119**

pen.k = pen.j +dt*(Desired Population - Population)^2: pen0 = 0. You don't need to perform the integration step in Coyle's formula, because Vensim integrates payoffs for you. The equivalent would be: *C population|desired population/weight If you make your weight something like PULSE( target_time, ...

- Thu Feb 18, 2010 4:42 pm
- Forum: Vensim
- Topic: Monte-Carlo output
- Replies:
**2** - Views:
**766**

- Thu Feb 18, 2010 4:40 pm
- Forum: Vensim
- Topic: Allocate Available function
- Replies:
**18** - Views:
**2707**

A basic assumption of the ALLOCATE logic, as I understand it, is that the quantities allocated are essentially an undifferentiated commodity. Given that the substitution relationships among resources are not symmetric, I think that assumption is violated. Therefore it does make sense to me to use a ...