The model seems to run fine for me - no overflow. Is there a particular test that's problematic?
One useful step at this point would be to get all your units balanced - that might reveal some hidden error in logic.
Tom
Search found 3816 matches
- Mon Apr 12, 2010 6:51 pm
- Forum: Vensim
- Topic: model initialization
- Replies: 2
- Views: 2367
- Wed Apr 07, 2010 1:31 am
- Forum: Vensim
- Topic: The question of optimization function
- Replies: 3
- Views: 3136
- Tue Apr 06, 2010 10:00 pm
- Forum: Vensim
- Topic: The question of optimization function
- Replies: 3
- Views: 3136
- Mon Apr 05, 2010 7:03 pm
- Forum: Vensim
- Topic: New user with newbie problems..please help
- Replies: 17
- Views: 11673
- Mon Apr 05, 2010 3:50 pm
- Forum: Vensim
- Topic: Rounding the value of levels.
- Replies: 3
- Views: 2936
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: 15027
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: 11673
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: 11673
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: 11673
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: 11673
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: 11673
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: 11673
- Thu Mar 25, 2010 3:03 pm
- Forum: Vensim
- Topic: Subscript ranges
- Replies: 2
- Views: 2190
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: 15027
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: 15027
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: 15027
- Wed Mar 24, 2010 9:04 pm
- Forum: Vensim
- Topic: Rainfall probabilities
- Replies: 30
- Views: 15027
- Wed Mar 24, 2010 4:17 pm
- Forum: Vensim
- Topic: Rainfall probabilities
- Replies: 30
- Views: 15027
- Tue Mar 23, 2010 2:43 pm
- Forum: Vensim
- Topic: Rainfall probabilities
- Replies: 30
- Views: 15027
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: 2153
- Wed Mar 10, 2010 12:14 am
- Forum: Vensim
- Topic: Optimize for particular value at a specific time
- Replies: 15
- Views: 7999
- Wed Mar 10, 2010 12:05 am
- Forum: Vensim
- Topic: using reality checks to build models
- Replies: 6
- Views: 5235
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: 3019
- Tue Mar 02, 2010 10:51 pm
- Forum: Vensim
- Topic: Optimize for particular value at a specific time
- Replies: 15
- Views: 7999
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: 2271