Whenever a service has a requirement for a resource, you have to choose the relevant basis for the requirement. In STEM versions 6.0–7.2 inclusive, the choices were:
-
Connections
-
Average Connections
-
Annual Traffic
-
Busy Hour Traffic
-
Annual Revenue.
Of these, the Connections,
Average Connections
and Busy Hour Traffic
bases were naturally end-of-period (or mid period) instantaneous results, whereas
Annual Traffic
and Annual Revenue
were defined in STEM 6.0 to represent the amount of traffic which would be carried in a year, even if the model were running in quarters or months, in order to avoid them being truly aggregate quantities. This was because the requirement relationship could only be interpreted in terms of demand for persistent capacity in versions of STEM prior to 7.3. For example, in a model running in quarters,
Annual Traffic
was calculated as Traffic × 4.
In STEM 7.3, this diversion is no longer necessary, and the more natural
Traffic
and Revenue
bases are re-instated, with the annual versions retained at the end only for backwards compatibility with existing models:
-
Connections
-
Average Connections
-
Traffic
-
Busy Hour Traffic
-
Revenue
-
Annual Traffic
-
Annual Revenue
-
New Connections.
Of these bases, just Traffic and
Revenue
are aggregate and represent the accumulated traffic carried or revenue earned in the period. It follows that these bases cannot be used directly to define requirements for the traditional persistent capacity of resources. The options for mapping these to consumable resources or converting to an annual rate are explained in the following sections.
The other place where the distinction between aggregate and instantaneous quantities is important is the Results Program:
- by default, instantaneous results are presented as line charts (connecting points in time), whereas aggregate quantities are displayed as column charts (period values)
- when quarterly or monthly results are consolidated as annual results, instantaneous results are typically selected from the last quarter or month in the year, whereas aggregate results are summed over each of the quarters or months in the year.
So consolidated Traffic or Revenue
results are summed over quarters or months.
Exactly the same options and distinctions apply when a basis is selected for a service input to a transformation, and similar caveats will apply to the use of transformations derived from an aggregate service input.
If a transformation has a resource input, then the following bases are available:
-
Installed Capacity
-
Installed Units
-
Used Capacity
-
Incremental Units
-
Incremental Capacity
-
Capital Expenditure
-
Utilisation
-
Consumption
-
Consumed Units.
Again, most of these measures are naturally end-of-period instantaneous results, with the exception of
Incremental Capacity, Incremental Units and
Capital Expenditure, all of which are evidently aggregate in nature and represent the accumulated capacity or units added or capital expended in a period.
With the exception of the new Time Factor
transformation described below, the output of a single-input transformation will be classified as aggregate if its input is.
In versions of STEM prior to 7.3 there was a single transformation
Output
result and, for historical reasons, this was always graphed and consolidated as an instantaneous (end of period) quantity. STEM 7.3 introduces a new transformation
Aggregate Count
result which exists alongside the original result (which has been renamed for clarity as
Instantaneous Output). Only one of these results will be available for any given element, according to whether its output has been classified as instantaneous or aggregate.
Note:
Instantaneous Output
is also available as Output for backwards compatibility.
In order to classify an expression (or sum) transformation, only certain combinations of instantaneous and aggregate inputs will be allowed (note: this is still work in progress):
-
all instantaneous or all aggregate
-
only instantaneous inputs referenced in the expression and only aggregate inputs used for cost allocation (i.e., with non-zero cost-allocation coefficients), or
vice versa
-
linear function of aggregate inputs divided by linear function of instantaneous inputs.