User Guide: C++ : Introduction to Statistics Gathering

10. Introduction to Statistics Gathering

CSIM automatically gathers and reports performance statistics for certain types of model components, including facilities and storages. CSIM also provides four general-purpose statistics gathering tools: tables, qtables, meters, and boxes. These tools can be used for the following purposes:

• to obtain statistics other than mean values for facilities and storages
• to obtain statistics for other model components, such as mailboxes and events
• to obtain statistics for selected submodels or for the model considered as a whole
• to employ the run length control algorithms provided with CSIM (see section 16.3, "Run Length Control")

Any statistics can of course be gathered by declaring and updating variables in a CSIM program. But, the statistics gathering tools are powerful and comprehensive, and their use will decrease the likelihood of programming errors that lead to incorrect statistics. Formatted reports of the statistics gathered with these tools can easily be included in the model output.

The following steps are suggested for adding statistics gathering to a model:

• Identify what statistics are of interest and which statistics gathering tools are appropriate.
• Declare a global pointer (variable) for each statistics gathering tool that will be used.
• Initialize each statistics gathering tool, usually at the beginning of the sim function.
• Add instrumentation (i.e., function calls) to the model to feed data to the tools.
• Generate reports by calling the report function.

The magnitudes of the performance statistics obviously depend on the time unit that is chosen for the model. Most of the reports produced by the statistics gathering tools will accommodate floating point numbers with six digits to the left of the decimal point and six digits to the right of the decimal point. Up to nine digits can be displayed for integer values. The time unit should be chosen to avoid performance values so far from unity that digits of interest are not displayed.

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