The Straight Line time-series lets you implement a simple linear growth using a known value and a constant. Its parameters may be readily linked to other inputs in order to adapt to a dynamic modelling context. The more extensible Interpolated Series time-series type is extremely flexible, but its individual periods and values cannot be linked to other inputs at present.
The straight line is defined using a Base value – a defined value of the straight line at a given period – and an Increment (growth value), which specifies the constant increase (or decrease) of the time-series over time (per year by default). Optional parameters are the Base Period, Increment Period and Constrain parameters.
Figure 1: Selecting the Straight Line time-series from the Type menu.
Base Period
The Base Period defines the time period when the Base value applies, similar to the Base Period input in the Exponential Growth and Floor and Multiplier time series. The Base Period can specify a month, quarter or year relative to the model start date (for example, Q3 or Y4), or a fully qualified date (such as 1 Jan 2000).
Increment Period
The Increment Period specifies the period over which the increment is applied (e.g., 1000 customers per Quarter), and may be Year (default), Quarter, Month, Week, Day, Hour, Minute or Second.
Constrain
If Constrain is set to Yes and if the Increment is positive, then STEM holds the output value constant at the Base value prior to the Base Period. This option provides a simple way to avoid troublesome negative values. Similarly, if Constrain is set to Yes and the Increment is negative, STEM holds the output value constant after the Base Period. If the Base value is negative, then Constrain will constrain the output to negative values, so that positive values are avoided.
If Constrain is set to No, the output is not constrained and maintains a constant Increment either side of the Base Period, whether or not any values are negative.
Figure 2: Using the Constrain input to avoid negative time-series values