POISSON DISTRIBUTION

[nikvyas]

Hello all.

I'm having a mental block with implementing a POISSON distribution within Vensim.

If the mean is 1.5, then (taken from Excel), the probability of:

1 is 22.3%
2 is 33.4%
3 is 12.5%

etc, etc.

Now, how do I represent this in Vensim please (obviously the mean will be variable)? The RANDOM POISSON equation (and all of its parameters) has got me a little bit confused unfortunately.

Any help gratefully received.

Thanks

Nik

[tomfid]

I think your Excel distribution is incorrect. From http://en.wikipedia.org/wiki/Gamma_function the density function is:

f(k,lambda) = lambda^k * EXP(-lambda) / k!

For k=1, lambda = 1.5, this looks like 1.5^1 * EXP(-1.5) / 1 = .33

I get the expected result from the attached model.

Hope this helps.

Tom

[nikvyas]

Thanks for that Tom - most useful.

And thanks for taking the trouble to add rainbow colours to your toolbar - glad i had a couple of Paracetamol to hand