A modeller uses the STEM Editor to create a model by arranging icons which represent model elements – market segments, services, resources, locations and various other functions – in a view, linking related elements together to represent the structure of their business and setting a range of parameters for each element to accurately describe the business. This model is saved to a pair of files – a .dtl file containing the internal model structure and data, and a .icp file that contains information required only for presentation to the user in the Editor (such as the screen position of each icon). A model may contain several named scenarios that define different values for a subset of the values in the model. It will always contain a base model known as the ‘working model’ from which all scenarios are derived.
A STEM model is run through the STEM calculation engine to generate a set of results based upon its values. The calculation is performed by a separate application that may be controlled directly by the STEM Editor, by other local Windows applications or via a COM interface. If successful, this generates one or more .smr results files. The results consist of arrays of numbers giving the value of a particular calculation at each requested period in time. (The model specifies a run period as a number of initial months, a number of quarters and thereafter a number of years.)
The primary means by which a modeller will view the model results is the STEM Results Program. This allows views to be created containing graphs or tables of results. It can perform further calculations to derive new results from those in the .smr file according to definitions specified in a .cnf configuration file which the modeller may edit and extend, either directly in a text editor, or with the Results Program.
STEM includes an Excel add-in that allows Excel to run models directly and to load model result values from a .smr file. It is also possible to include formulae within a STEM model that link input values from an Excel file. The STEM Editor and Results Program include functionality to generate these links to Excel which may also be edited manually by the user. Thus Excel can be used as a simplified front-end for a model, including pre-processing of inputs or further post-processing of native STEM results.