Ventana software support forum

tomfid wrote: ↑Sat Oct 17, 2020 3:18 pm That would be:
EXP( RANDOM NORMAL(-100,-6.05,-13.3,3.49,stream) )
where -6.05 = ln(2.35e-3)

The -100 is just a big number (30 standard deviations roughly) - the truncation at 0 isn't really needed since the lognormal is always >0.

Hi tomfid
thank you a lot.
All the best.
Lorenzo

Hi tomfid
could you be more clear about this: "the maximum departure in terms of number of geometric std dev above the mean"?
Indeed I have to generate a lognormal whose normal parameters are μ = −13.3 and σ = 3.49 truncated on the interval [0, 2.35 × 10−3]).
Lorenzo

Hi tomfid
is it the same if I use this form?
randx = EXP( normal_mean+normal_std_dev*RANDOM NORMAL(-100, LN(max val),0, 1, 0 )
Lorenzo

The RANDOM NORMAL function generates a truncated Normal distribution, so if you transform that you'll get a truncated Lognormal. You could do something like: randx = EXP( RANDOM NORMAL(-something large, LN(max val), LN(geometric mean), geometric SD, stream ID) ) Hi tomfid thank you a lot... fast an...

Hi everybody is there any way to use, in a Vensim model, a variable that simulates a truncated lognormal? I know how I can simulate a "full" lognormal through a normal distribution but what about the insertion of a maximum positive value? How can I rescale so that the area under the trunca...

Hi tomfid
thank you.. very simple, indeed, like Columbus' egg...
very precise and accurate as usual.
all the best.
Lorenzo

Hi
you can check the "Show behavior" item under the view menu or you can select the desired variable and then left click on the item Graph on the left side of the Vensim main window...
All the best.
Lorenzo

Hi all I would like to use a parameter modelled with a Frechet distribution in a MonteCarlo simulation of a Vensim Plus model. In SntheSym I can use the trick of implementing the Frechet distribution as betaScale*(-LOG(X , 10 ))^(-1/alphaShape) but how can I proceed in the Sensitivity Simulation Set...

Even on a Mac you should be able to print the currently open model in Vensim on a file with the proper resolution (300 dpi or higher).
All the best.
Lorenzo

Hi tomfid
you hit the point....I suggest this as an extension for a future version of Vensim.
All the best.
Lorenzo

Hi tomfid
thank you for your (according to me wrong) answer... if ideed x spans over the interval (0,100) you get also negative values and not a logaritmic scale.
All the best.
Lorenzo

Hi everybody
is there any way to have, in Vensim PLE, an x,y diagram with x in Ln scale? Please do not suggest to use Ln(x) sice it does not work...
I mean a true logaritmic scale...
All the best.
Lorenzo

Hi tomfid
thank you a lot... I'll stand on waiting...are you going to put the pdf file on the Ventity web-site?
Lorenzo

Hi all
I'm a newbie of Ventity and I prefer old style documentation... is there any PDF-style tutorial somewhere around?
All the best.
Lorenzo

Hi JJ
the implementation in Vensim as
betaScale*(-LOG(X , 10 ))^(-1/alphaShape)
with X as RANDOM UNIFORM(0, 1, 0) is not robust since if X is about 1 (for instance 0.999223) I get a value of 5.32112e+006
Any idea?
All the best.
Lorenzo

LAUJJL wrote:Hi

If you type Fréchet distribution in google, Wikipédia explains how to get a Fréchet distribution from the uniform (0,1) distribution.

Regards.

JJ

Hi JJ
thanks a lot.
Lorenzo

LAUJJL wrote:Hi

If you type Fréchet distribution in google, Wikipédia explains how to get a Fréchet distribution from the uniform (0,1) distribution.

Regards.

JJ

Hi JJ
thanks a lot.
Lorenzo

Hi
is there any way to simulate in Vensim a Frechet distibution with parameters alpha or shape and beta or scale?
Lorenzo

There are two different parameterizations of the exponential in common use, one with f(x,a) = a*EXP(-x*a) and the inverse, f(x,b) = 1/b*EXP(-x/b) So, we can't reliably answer without knowing which version your alpha corresponds with. Assuming that you're expecting a mean of 1/alpha, your suggested ...

I would set m and x to 0 and a very large value, unless you want to truncate the distribution. I think the mean adjustment is correct, but as a test, you should generate a stream of numbers with your proposed distribution, and check them, either with the Stats tool and Bar chart with histogram opti...

The Weibull is normally a 2-parameter distribution, so I'm not sure how the gamma/location parameter should be interpreted. I think S = shape = alpha is a safe bet. However, the Weibull is often parameterized so that mean = scale * Gamma( 1 + 1/shape ) whereas Vensim is scaled so that mean = 1. So,...

Administrator wrote:How are alpha, beta and gamma defined in the other software you are using?

Hi Administrator
you are right, sorry, I forgot specifying...
alpha shape
beta scale
gamma location parameter

so I would guess
S=alpha
h=gamma
r=beta
Am I right?
Thank you a lot.
Lorenzo

Hi everybody on the Vensim's manual you can read RANDOM WEIBULL(m,x,S,h,r,s) WEIBULL with shape S starting at 0 with mean 1 and RANDOM WEIBULL(m,x,S,h,r,s) provides a Weibull distribution with shape S starting at 0 and having a mean of 1 before it is stretched, shifted and truncated. When S is 1 the...

Priscila wrote:Hello,

I would like to know if there is a way to export from vensin only the results of only one variable.

Anyway if you export in either a .tab or a .xlx file you can use excel to gather only the data of the preferred variable....
Lorenzo

Administrator wrote:GET XLS LOOKUPS is the function that does this.

Hi Administrator, thank u a lot, it works really fine..
Lorenzo