Sensitivity analysis input

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YahyaRafique
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Sensitivity analysis input

Post by YahyaRafique » Mon Sep 20, 2010 2:27 am

Hi all,

I have purchased the PLE Plus version of Vensim.

My objective is to evaluate the "degree of influence" of each parameter in the sensitivity analysis on one of of my output variables.

Unfortunately the statistical software that I have does not support the input of parameter values, generated by Vensim based on the probability distributions.

Now my obstacle is figuring out how I can input values, generated by my stats tool, into Vensim i.e. I don't want Vensim to generate values but to obtain them from an exteral input. I regard the reference manual regarding importing data values for a single simulation. But this approach is rather time consuming since I have to manually run the simulation each time and record the output.

I was wondering if there is a work around for this scenario.

Regards,

Yahya

Administrator
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Post by Administrator » Mon Sep 20, 2010 6:49 am

Unfortunately the statistical software that I have does not support the input of parameter values, generated by Vensim based on the probability distributions.
Why is this? Can you just convert them somehow so they match the format you need?

Now my obstacle is figuring out how I can input values, generated by my stats tool, into Vensim i.e. I don't want Vensim to generate values but to obtain them from an exteral input. I regard the reference manual regarding importing data values for a single simulation. But this approach is rather time consuming since I have to manually run the simulation each time and record the output.
Command files would probably help you here, but I'm pretty sure you cannot use them in PLE Plus. Do you have access to Vensim DSS at all?

Tony.

bob@vensim.com
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Post by bob@vensim.com » Mon Sep 20, 2010 9:27 am

There is an option in sensitivity to use a file for the input parameters to a sensitivity simulation. Details in Chapter 10 of the Reference Manual.

YahyaRafique
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Post by YahyaRafique » Mon Sep 20, 2010 8:27 pm

@Administrator:

There is no support in the software for reading parameters values, it only allows reading of output from the simulation. Hence I have to try to reach a workaorund in Vensim. I don't have access to Vensim DSS which is why this is an issue. The software I am using for stats purposes, Simlab, is integrated with MATLAB so I wondering if I can possibly link Vensim with MATLAB, again keeping in mind that the version of Vensim I have is PLE PLus.

@bob:

I agree that in Chapter there are details of entering input parameters for a sensitivity analysis but again using this approach I can only specify a probability distribution from which variable values will be generated by Vensim. My question is whether I can enter for example 1000 sets of variable values for 1000 simulation runs.

Thanks to both for the feedback

[Edited on 20-9-10 by YahyaRafique]

tomfid
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Post by tomfid » Mon Sep 20, 2010 11:00 pm

You can use a file containing an actual sample of input parameters, not just distributions. You specify the name of the file in the sensitivity control file. Take a look again at Chapter 10 of the reference manual, e.g.

File (use a file to specify parameters), if selected, will use a file to determine what inputs to change, and ignore any constants specified in this dialog. This file must be a tab delimited text file (UTF8 for international characters). The first row should contain a list of model constants (tab delimited), with the values on each successive row. If a value is missing from this file the previously used value is repeated.

Tom

YahyaRafique
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Post by YahyaRafique » Thu Sep 23, 2010 2:40 am

Works like a charm! Thanks guys for the help :-)

mmt
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Re: Sensitivity analysis input

Post by mmt » Tue Jun 28, 2011 7:42 pm

Hello,

Following the instructions on this page I am trying to run sensitivity analysis using input file (Tab delimited, UTF-8 encoded), however somehow I am unable to get Vensim to read the file.
I get an error 'BAD CONSTANT' on compilation. Can you advice where I am going wrong. Would be great if you could post a sample input file.
Thanks in advance.

Administrator
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Re: Sensitivity analysis input

Post by Administrator » Tue Jun 28, 2011 10:14 pm

Sounds like you are either trying to modify something that is not a constant, or have a typo in your input file.

Can you post it and the model so we can take a look?
Advice to posters seeking help (it really helps us to help you)
http://www.ventanasystems.co.uk/forum/v ... f=2&t=4391

Units are important!
http://www.bbc.co.uk/news/magazine-27509559

mmt
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Re: Sensitivity analysis input

Post by mmt » Wed Jun 29, 2011 12:44 pm

Hello,

Thanks for the reply. I have attached a sample file. This is a simple queue model with an arrival and departure rate.
I wish to run numerous (1000+) instances of this model with different values of the arrival rate. Using the instructions in manual chapter 10
I have created a sample input file. The syntax suggested in the manual is as under.
#simulations,U|M|L,seed
constant=distribution(min,max,...)

On running the simulation I get a list of 'bad constant' errors. I think that I have not understood the format of the file properly. Also I have used a 'Vector' function in the file.
But instead I intend to input specific values of arrival rate without any min,max and step. I am not sure how to do that.
Please do advice. Thanks again for your support.
Attachments
Queue.zip
(2.41 KiB) Downloaded 82 times

tomfid
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Re: Sensitivity analysis input

Post by tomfid » Wed Jun 29, 2011 4:04 pm

Using the "File" approach to sensitivity setup, you don't specify distributions; you specify the actual sample to be tested as a table of values. So, if you wanted to do that, your .tab file would look like:

param1<tab>param2
1.1<tab>2.4
3.9<tab>2.2
etc.

If you want to use a distribution, like VECTOR, you specify that directly in the .vsc, and use one of the other combinatorial options (univariate, multivariate, latin ...). Your .vsc setup will look like the dialog screenshot here:
http://www.vensim.com/documentation/ind ... ?23530.htm

mmt
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Re: Sensitivity analysis input

Post by mmt » Wed Jun 29, 2011 5:24 pm

Thanks a lot for your help !!

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