Search found 3811 matches
- Thu Aug 28, 2014 4:57 pm
- Forum: Vensim
- Topic: Need help with formulas
- Replies: 26
- Views: 13152
Re: Need help with formulas
Steady state error = in equilibrium, actual workforce (cumulative workforce level) doesn't equal required workforce. If you simplify to a first order (one stock model), it's easy to see why this happens: workforce = INTEG( hiring - departing, initial workforce ) hiring = (desired workforce - workfor...
- Thu Aug 28, 2014 2:53 pm
- Forum: Vensim
- Topic: Need help with formulas
- Replies: 26
- Views: 13152
Re: Need help with formulas
Take a look at http://vensim.com/causal-tracing-in-vensim/ , then revisit your model. The aggregate behavior looks pretty reasonable to me, as long as willingness to increase workforce = 1. There's some steady state error (equilibrium workforce is short .3 people), because the hiring decision doesn'...
- Wed Aug 27, 2014 7:03 pm
- Forum: Vensim
- Topic: Need help with formulas
- Replies: 26
- Views: 13152
Re: Need help with formulas
There are two ways to isolate: - copy/paste the structure into a separate model, improve it until you are satisfied, then copy/paste back into the complete model, or - break the causal links between the elements of the integrated model temporarily One way to implement the second approach might be: {...
- Wed Aug 27, 2014 2:22 pm
- Forum: Vensim
- Topic: Need help with formulas
- Replies: 26
- Views: 13152
Re: Need help with formulas
It's often helpful to isolate the sectors of the model (for example, rework cycle and personnel skills), so that they can be tested independently and refined before they are integrated.
- Wed Aug 27, 2014 2:19 pm
- Forum: Vensim
- Topic: Model on Schedule Pressure
- Replies: 3
- Views: 2888
Re: Model on Schedule Pressure
A good guideline for modeling is "always be done" - that is, don't add new structure until the existing structure is working and understood. Given that you have questions on a number of structures still, I'd recommend holding schedule pressure constant for now until those are resolved. The...
- Wed Aug 27, 2014 4:07 am
- Forum: Vensim
- Topic: Unable to open the file ReferenceMode.vdf.
- Replies: 13
- Views: 14117
Re: Unable to open the file ReferenceMode.vdf.
This is a bug that's triggered by using the Reference Mode tool, but not entering any data. You can remove the reference by saving the model as text (.mdl) and editing it with a text editor to remove the ReferenceMode.vdf lines near the end, like: ///---\\\ :GRAPH Avg :TITLE Avg :SCALE :VAR random x...
- Wed Aug 27, 2014 3:55 am
- Forum: Vensim
- Topic: Data Only Modeling
- Replies: 5
- Views: 2997
Re: Data Only Modeling
Strangely, I can now open the first version of your model. If you want it to run, you need to visit each equation in the editor, hit OK, and say Yes when Vensim offers to update the input list. This will leave you with a model that has no causal arrows, and therefore no dynamics - not very interesti...
- Wed Aug 27, 2014 3:48 am
- Forum: Vensim
- Topic: Completion Tasks not varying
- Replies: 9
- Views: 5903
Re: Completion Tasks not varying
Here's how I think of this (hopefully it corresponds with your situation): A released program or module is essentially a compiled snapshot of a codebase at some point in time. It inherits the errors, completeness, and other properties of the codebase at the time of release. Normally, a module is rel...
- Wed Aug 27, 2014 3:40 am
- Forum: Vensim
- Topic: Data Only Modeling
- Replies: 5
- Views: 2997
Re: Data Only Modeling
Not so fast ... You can certainly use data to drive the behavior of your model. You just can't have a variable simultaneously (a) show causal arrows on the diagram and (b) contain only data. A data variable can have a) no equation, in which case it must be supplied in an input data file (vdf) b) a G...
- Sun Aug 24, 2014 11:37 pm
- Forum: Vensim
- Topic: count number of people and set variable from 0 to 1
- Replies: 4
- Views: 2522
Re: count number of people and set variable from 0 to 1
It sounds like you need a stock that serves as a counter. Whenever the research loop is triggered, you flush out the contents (outflow = stock/TIME STEP) and start counting again.
- Sun Aug 24, 2014 9:25 pm
- Forum: Vensim
- Topic: Completion Tasks not varying
- Replies: 9
- Views: 5903
Re: Completion Tasks not varying
In this version, it looks like "completing tasks" is always 0 due to the (1-effect of misunderstanding...) factor in productivity.
- Sun Aug 24, 2014 6:45 pm
- Forum: Vensim
- Topic: Enable the TIME STEP from Model Settings
- Replies: 2
- Views: 2126
Re: Enable the TIME STEP from Model Settings
The field could become non-editable if you made the time step a variable expression. Add TIME STEP to your diagram as a shadow variable, and edit it from that.
- Sun Aug 24, 2014 6:44 pm
- Forum: Vensim
- Topic: Data Only Modeling
- Replies: 5
- Views: 2997
Re: Data Only Modeling
I can't open the file you posted, but judging by the text, demolition=A FUNCTION OF(demolition,house typology,housing stock,tenancy) ~~| demolition:INTERPOLATE: ~ buildings ~ | You have causal arrows connected to your data variables. You can't have it both ways - if an equation is driven by data, it...
- Sat Aug 23, 2014 10:21 pm
- Forum: Vensim
- Topic: Confused about the variable to be used.. Plz help
- Replies: 3
- Views: 2502
Re: Confused about the variable to be used.. Plz help
If modules are discrete entities, it seems like you'd need a copy of the task structure for each module. Otherwise you can't distinguish between making 5 modules 10% more complete and making 1 module 50% more complete. If there are just a few modules, you could handle this with copy/paste, but other...
- Sat Aug 23, 2014 10:09 pm
- Forum: Vensim
- Topic: Reworks in tasks
- Replies: 17
- Views: 8290
Re: Reworks in tasks
Actually it might be better to consider the release as a "copy" rather than a "move" - something like the following:
- Fri Aug 22, 2014 6:55 pm
- Forum: Vensim
- Topic: Confused about the variable to be used.. Plz help
- Replies: 3
- Views: 2502
Re: Confused about the variable to be used.. Plz help
See my comment on the other thread.
- Fri Aug 22, 2014 6:49 pm
- Forum: Vensim
- Topic: Reworks in tasks
- Replies: 17
- Views: 8290
Re: Reworks in tasks
If I understand your problem framing, I think you'd want something like the following structure. The release switch could be something like a PULSE, if it happens at a particular time, or an IF THEN ELSE statement if it's conditional on the state of the project. The equations for the "releasing...
- Fri Aug 22, 2014 6:37 pm
- Forum: Vensim
- Topic: Vensim debugging
- Replies: 22
- Views: 11958
Re: Vensim debugging
Since there's no % type or display mode in Vensim, 99~percent and 0.99~dmnl should be different. To avoid percent/fraction conversion errors, I generally create a unit converter variable, "100 percent"==100~percent, and use that. For example: percent real fruit juice = "100 percent&qu...
- Wed Aug 20, 2014 7:53 pm
- Forum: Vensim
- Topic: Tasks perceived completed and inexperienced personnel
- Replies: 4
- Views: 2972
Re: Tasks perceived completed and inexperienced personnel
"Structure" in this context means that if QI must be 0 to 1, the equations that are inputs to QI, and the form in which they are combined, must always produce a result that is 0 to 1. Example 1 (bad): QI = a*schedule_pressure + b*overtime There's no guarantee in general that this will yiel...
- Wed Aug 20, 2014 7:45 pm
- Forum: Vensim
- Topic: Using correlation values obtained from SPSS on Vensim
- Replies: 10
- Views: 5339
Re: Using correlation values obtained from SPSS on Vensim
Sensitivity analysis will vary the values across simulations, not over time.
- Wed Aug 20, 2014 7:35 pm
- Forum: Vensim
- Topic: Reworks in tasks
- Replies: 17
- Views: 8290
Re: Reworks in tasks
I'm unclear on the meaning of the flow from "tasks done" to "work accomplished in module" - is this some kind of release process, where tasks done are declared to be part of a finished module at some point?
- Wed Aug 20, 2014 6:24 pm
- Forum: Vensim
- Topic: Problems with "Find zero" convergence
- Replies: 13
- Views: 6177
Re: Problems with "Find zero" convergence
An implicit assumption in the FIND ZERO function is that the response of the vector to be zeroed to the control vector to be set meets some reality checks. For example, it needs to be continuous and have a gradient everywhere. Otherwise the algorithm can easily get stuck on flat spots, local minima ...
- Wed Aug 20, 2014 6:15 pm
- Forum: Vensim
- Topic: Problems with "Find zero" convergence
- Replies: 13
- Views: 6177
Re: Problems with "Find zero" convergence
In this case, the scaling factors should not be 1: zs is a scaling vector for z. After multiplying z by this vector the results should all be of the order of magnitude of 1. zs must be a vector of the same length as x and will often be a vector of ones. http://www.vensim.com/documentation/fn_find_ze...
- Wed Aug 20, 2014 6:05 pm
- Forum: Vensim
- Topic: Output Custom Graph
- Replies: 6
- Views: 4537
- Wed Aug 20, 2014 3:50 pm
- Forum: Vensim
- Topic: Gamma Density curve?
- Replies: 2
- Views: 2045
Re: Gamma Density curve?
There's a RANDOM_ function for generating from the gamma distribution, if that's what you need. There's also the GAMMA_LN function that gives you the log of the gamma function, which can be used to calculate the gamma pdf.