Search found 1421 matches

by LAUJJL
Wed Nov 08, 2006 5:34 pm
Forum: Vensim
Topic: exponential distribution
Replies: 14
Views: 6290

exponential distribution

Hi It is always the same kind of problem: continuous versus discreet. In continuous environment where the time is sliced and the data aggregated during the time step, it is diffucult to represent an event. In fact you should consider a sufficiently small time step and in this time to simulate the re...
by LAUJJL
Wed Nov 08, 2006 3:26 pm
Forum: Vensim
Topic: exponential distribution
Replies: 14
Views: 6290

exponential distribution

Hi

joined a simplistic model that has exponential failures along the time axis.
Note that the exponential distribution is continuous and it is necessary if the amount of failure is small to convert the continuous value into a discreet one, using the integer value.
Regards.
JJ
by LAUJJL
Wed Nov 08, 2006 1:23 pm
Forum: Vensim
Topic: exponential distribution
Replies: 14
Views: 6290

exponential distribution

the exponential distribution is valid only if the probibility of failure is independant of the age of the equipment and is not valid if it increases when the equipment grows older.
The normal, log normal are more valid in this case.
JJ
by LAUJJL
Wed Nov 08, 2006 1:07 pm
Forum: Vensim
Topic: exponential distribution
Replies: 14
Views: 6290

exponential distribution

Hi
the exponential distribution is effectively used to model the
number of failures during a certain time among a certain number of machines mong other distributions.
Regards.
JJ
by LAUJJL
Wed Nov 08, 2006 12:36 pm
Forum: Vensim
Topic: exponential distribution
Replies: 14
Views: 6290

exponential distribution

the poisson (with peaks or not) distribution is currently used to model the new incomers in a queue or the failures of a set of approxiamtely anything, and the exponential distribution is used to model the time of repair or servicing the customer in the queue, at least in the litterature. Regards. JJ
by LAUJJL
Wed Nov 08, 2006 8:59 am
Forum: Vensim
Topic: find zero
Replies: 24
Views: 12118

find zero

Hi Peter You should, before sending a model, both checks, the equations and the units. Both have mistakes. There are no values and no units for the derivatives. The price market equal to exp(- market share of a - market share of b) is equal to exp (-1) that is constant. or the price will vary with t...
by LAUJJL
Tue Nov 07, 2006 11:36 am
Forum: Vensim
Topic: find zero
Replies: 24
Views: 12118

Find ZEro

Hi Here is joined a model with units. I had to modkfy the equations that were highly inconsistent. It has still problems because the find zero is not included in a loop without stocks. It cannot loop to find the optimal solution. you should study closely the prion example, to have an example of use ...
by LAUJJL
Tue Nov 07, 2006 9:00 am
Forum: Vensim
Topic: find zero
Replies: 24
Views: 12118

Find ZERO

Hi Peter The variables you have are not dimensionless because if you put dmnl as dimension, the model will not stand the unit checks. Dimensionless and no dimension at all is not the same thing. I think that one of the force of SD among a lot of drawbacks is the capability to dimension the values to...
by LAUJJL
Mon Nov 06, 2006 5:14 pm
Forum: Vensim
Topic: find zero
Replies: 24
Views: 12118

Find zero

Hi Peter

Can you put the units in your model.
It may help you more than me.
If you have still problems, I will try to help you.
Regards.
JJ
by LAUJJL
Mon Nov 06, 2006 4:57 pm
Forum: Vensim
Topic: how to set up the units in vensim
Replies: 4
Views: 2664

Units

Hi

I do not see any unit errors in the model attached.
For your first question, go to the reference manual and look at the second chapter about the Vensim language. It will explain the order taken by Vensim to make calculations.
You should too study the Vensim user's Manual.
Regards.
JJ
by LAUJJL
Sun Nov 05, 2006 9:27 am
Forum: Vensim
Topic: find zero
Replies: 24
Views: 12118

find zero

Hi Addor

I cannot see any attachment in your post.
It would be a good idea to join a simple model to illustrate your difficulties!
Regards.
JJ
by LAUJJL
Sat Nov 04, 2006 10:09 am
Forum: Vensim
Topic: Point of this forum
Replies: 6
Views: 3749

Point of this Forum

Hi Nick About the specificities, I mean the specificities of the problem. If only generalities mattered, you would find on the market plenty of SD model already made with parameters for the specificities. This is not the case and I think this is one of the reasons of the difficulty of SD to be more ...
by LAUJJL
Fri Oct 27, 2006 4:19 pm
Forum: Vensim
Topic: Point of this forum
Replies: 6
Views: 3749

number of time steps

Hi Nick I prefer to talk about concrete problems than about generality, because problems are always different. So to explore your problem it would be better to expose it in a more concrete manner, for instance your model or something that is like it. The 8000 time steps is just an upper limit that i...
by LAUJJL
Thu Oct 26, 2006 3:36 pm
Forum: Vensim
Topic: Point of this forum
Replies: 6
Views: 3749

point of this forum

Hi Nik I always try to help people, as I think that if you want to be helped sometimes, one has to help in return. But there are four cases where I do not reply. First when I do not feel competent. Second if I feel that the question is somewhat absurd. Third if I notice that the user never answers t...
by LAUJJL
Tue Oct 10, 2006 7:50 am
Forum: Vensim
Topic: IF THEN ELSE Qns
Replies: 5
Views: 3498

nested if then else

Hi
To avoid the syntax, the solution is to use intermediary variables.
If then else (A < B,C,D),
D = if then esle (X >Y,E,F)
It reduces the complexity of the equations but increases the
complexity of the sketch.
Regards.
by LAUJJL
Tue Oct 10, 2006 7:34 am
Forum: Vensim
Topic: Asset Maintenance Model
Replies: 5
Views: 3766

Asset maintenance model

Hi Melvyn

It would be a good idea to post your model, because I do not understand how you represent reality in your model.
It is difficult to give any advice in this case.
Regards.
JJ
by LAUJJL
Sat Oct 07, 2006 10:39 am
Forum: Vensim
Topic: LOOKUP
Replies: 3
Views: 2396

variable lookup

Hi Cathy A lookup is a function that is not supposed to change during the course of a simulation, as far as I know. The only solution that I know to have a changing lookup is to define the function by yourself. For instance, if you have a very simple lookup where Y is the output and X the input wher...
by LAUJJL
Fri Sep 22, 2006 5:21 pm
Forum: Vensim
Topic: Too many (32766) loops - truncating to 8000
Replies: 5
Views: 3172

too manay loops

Hi

Can you post your model, so we can all see what your model is looking like. Maybe we can help if we can see it.
Regards.
JJ
by LAUJJL
Fri Sep 22, 2006 2:55 pm
Forum: Vensim
Topic: Too many (32766) loops - truncating to 8000
Replies: 5
Views: 3172

too many loops

Hi The software is right, when it says that there are too many loops for only one variable, at least to list them, because the model may still run correctly. Anyhow, what is the interest to list them, because I would like to know the method you will follow to analyze all these loops! Regards. Jean-J...
by LAUJJL
Wed Sep 13, 2006 8:08 am
Forum: Welcome and General Information
Topic: updating the sd mailing list archive
Replies: 1
Views: 15103

updating the sd mailing list archive

Hi

What about updating the sd mailing list archive that is now more then two months old?
I have unsubscribed from the Sd mailing list because of too much work but still like to have a look at the past discussions.
Regards.
JJ
by LAUJJL
Tue Aug 15, 2006 12:27 pm
Forum: System Dynamics Discussion
Topic: System Dynamics in Business
Replies: 6
Views: 5761

Several years SD takes in business

Hi Ralf I think that the time it takes to get any positive results from SD depends on a lot of factors. Mainly the kind of subject, the expertise of the modellers, the knowledge of the subject from the people dealing with it, the ability of the organisation to implement new methods, the state of the...
by LAUJJL
Sun Aug 13, 2006 11:45 am
Forum: Welcome and General Information
Topic: wrong dates
Replies: 1
Views: 13990

wrong dates

Hi

dates are incorrect and need to be changed.
Regards.
JJ
by LAUJJL
Tue Aug 01, 2006 10:34 am
Forum: System Dynamics Discussion
Topic: System Dynamics in Business
Replies: 6
Views: 5761

Lean thinking

Hi Lean thinking cannot be considered as a paradigm (a scientific or sometimes not so scientific, methodology to explore the world) but the basis of human progress towards more value with less effort that has governed the world since the beginning of humanity. So lean thinking is not new, but has be...
by LAUJJL
Mon Jul 24, 2006 12:50 pm
Forum: Vensim
Topic: multiple input
Replies: 3
Views: 2664

RE I have a question

Hi

I do not understand your question.
I think it needs a better formulation.
Regards.
JJ
by LAUJJL
Sun Jul 23, 2006 3:40 pm
Forum: Vensim
Topic: Need Help...
Replies: 2
Views: 2283

iteration in a time step

Hi One way is to build variables subscripted with the dimension of the subscript approximately equal to the number of iterations expected and to make the variables equal to the values of the variables for each iteration. joined a simplistic example that computes the limit of 1 / 2N when N is increas...