Stat dist

rezakazemi · Post by **rezakazemi** » Wed Feb 25, 2009 4:46 am

Is Lognormal distribution available in Vensim?

bob@vensim.com · Post by **bob@vensim.com** » Wed Feb 25, 2009 11:40 am

I assume you mean with random functions. Just use exp(RANDOM NORMAL(nmean,nstdv,0)). I think, though it is worth double checking, that the mean for this will be

mean of log normal = exp(nmean + nstdv^2/2)

and the variance will be

variance of log normal = exp(2*(nmean + nstdv^2)) - exp(2*stdv + nstdv^2)

Hope that helps.

Danny · Post by **Danny** » Fri Jul 10, 2009 7:26 am

I wanted to use the suggested formula but I always get a floating point error.
What can be the reason?

Thanks, Danny

bob@vensim.com · Post by **bob@vensim.com** » Fri Jul 10, 2009 9:34 am

most likely your mean and standard deviation put the values out of range. these need to be pretty small numbers since you are passing them through exp. Try with a mean of 0 and a standard deviation of 1.