An S-shaped curve, or logistic, is traditionally used to model the take-up of a service: slow growth at first, followed by a period of more rapid growth, before levelling off to a saturation value. The series is uniquely defined by the following:
Saturation
The asymptotic value of the series, as time either increases or decreases, according to the relative sizes of Value A and Value B.
Base period
A calibration period, before and at which the series is constant at zero, or the Saturation level, again according to the relative sizes of Value A and Value B.
Value A in Period A and Value B in Period B
Two specific reference points for the series. STEM fits an S-shaped curve through these two points to match the Saturation level and Base Period. If the first value is greater than the second, you can define a curve that starts at the Saturation level, and then falls, representing a declining Service.
Options
Birth & Death Periods which enable you to ‘switch’ the curve on and off (from zero).
The value in year n, Tn, is calculated as:
for n > c
where
s = Saturation
c = Base Period
a = a constant
b = a constant
and 
Given the complexity of the formulae above, the Editor graph facility is particularly useful for previewing the interpretation of S-Curve parameters.
Displaying a graph of an S-Curve time-series input