S-Functions and modeling user growth

[busboy10]

Hi All,

I am trying to model user adoption based on a s-curve adoption rate
for cities. So I have a simple s-function that shows town growth and
I want to create a S-function for user growth based on the number of
towns.

The problem I am finding is that when the town jumps from let's say 1
to 2, the user growth now needs essentially two different formulas.
One to continue the growth on the first town and now a new s-formula
for the new town. This will continually happen as the towns continue
to increase.

Any thoughts on how to do this. I was thinking I could do it in VBA
but I am not sure where to start. I am thinking I could use some kind
of recursive function but am unclear if that is the best way.

Any thoughts?

Thanks!