PARAMETRIC ANALYSIS VIA THE ARENA PROCESS ANALYZER

9.7 PARAMETRIC ANALYSIS VIA THE ARENA PROCESS ANALYZER

The term parametric analysis refers to the activities of running a model multiple times with a different set of input parameters for each run, and then comparing the resultant performance measures. The purpose of parametric analysis is to understand the impact of parameter changes on system behavior (sensitivity analysis), often in the process of seeking the optimal configuration (parameter set) with respect to one or more performance measures or combination thereof.

In Arena parlance, input parameters are called controls, and the resultant perform- ance measures are called responses. A collection of controls and responses for a given set of runs is referred to as a scenario, and a collection of scenarios is termed an Arena project.

The Arena Process Analyzer is a tool that supports parametric analysis of Arena models by allowing the modeler to create, run, and compare simulated scenarios, and thus observe the effect of prescribed controls on prescribed responses. Controls may consist of variables and resource capacities, while responses include both variables and statistics (note that only variables can serve both as controls and responses). User- defined categories of controls and responses are also allowed by using modules from the Elements template panel, such as Resources, Variables, Tallies, Dstats, Counters, and Outputs.

The Process Analyzer is accessible via the Windows Programs option or from the Tools menu in the Arena home screen. Once an Arena model is created, debugged, and validated, selecting the Process Analyzer option pops up a separate Process Analyzer window for each Arena project. To create scenarios, the modeler selects the New option in the Process Analyzer File menu to pop up a grid (table) panel for specifying Arena scenarios and their constituent controls and responses—one scenario per grid row.

Figure 9.16 displays a Process Analyzer window with an initial grid (identified by the heading Scenario Properties) and a panel to display charts below it. A new scenario specification is added either by selecting the Scenario. . . option in the Insert menu or by double-clicking below the grid as indicated by the Arena message. Each scenario specification consists of the following columns:

1. The S column is check-marked by Arena when a scenario is valid, and flagged when the scenario run is completed.

2. The Name field assigns a user-defined name to the current scenario.

3. The Program File field specifies a file name (.pan file), in which Arena saves user-defined scenarios. This file is created by Arena automatically whenever the underlying model is checked for correctness via the Check Model option in the Arena Run menu. The generated .pan file is placed in the same directory as the underlying .doe model file.

4. The Reps field specifies the number of replications to be run for the current scenario.

5. Additional columns for specifying controls and responses may be added to the grid by selecting the Control. . . or Response . . . option, respectively, in the Insert menu. These are inserted to the right of the existing columns in the order of creation. Arena then opens a dialog box with a tree of available controls or responses, and the modeler navigates the tree and selects the desired option.

Output Analysis 191

Figure 9.16 A Process Analyzer window with an initial grid.

To the left of the grid panel is the Project bar, used to display project information. The Project bar has two tabs at the bottom: the Project tab displays information on grid rows (scenarios), grid columns (controls and responses), and graphical charts, while the Status tab displays information on the current scenario being run.

Once the grid data are specified, the modeler may proceed to run the scenarios in the current Process Analyzer project. Individual scenarios can be run one by one by highlighting each requisite scenario in the grid, and then selecting the Go option in the Run menu. To run multiple scenarios, the modeler simply highlights the requisite set of scenarios (using the Ctrl key), and then launches the run. The scenarios will be run one after the other without user intervention. All Process Analyzer replications run in batch mode (without animation). As each scenario run is completed, the corresponding response values appear in the grid under the Response columns, replacing the initial dashes there. The resulting grid is handy for output analysis comparisons of project scenarios. Once a scenario run terminates, the modeler may visualize response statistics by creating charts. Charts can be defined using the Chart. . . option of the Insert menu.

The Process Analyzer facilitates sensitivity analysis, especially in complex systems where there may be many controls, responses, replications, and scenarios that have long execution times. As an example of a typical use of the Process Analyzer, consider a

192 Output Analysis single-machine workstation with random job arrivals and a finite buffer capacity. Jobs

arriving at a full system are considered lost. We wish to perform a parametric analysis to understand the impact of the buffer capacity on system performance. To this end, we select the buffer capacity to be a control, and let the corresponding responses be buffer- related statistics, including the probability that the buffer is full, average WIP level, average delay in the buffer, as well as the system throughput.

The parametric analysis plan is as follows. In each successive scenario, the buffer capacity is increased by unity. In consequence, we expect the probability that the buffer is full to decrease as the buffer size increases, so that more jobs can enter the system. Therefore, the average WIP level and the throughput should increase.

Figure 9.17 depicts a Process Analyzer window with a populated grid. The Project bar shows that five scenarios have been defined: one control (buffer capacity) and four responses (the statistics mentioned above). However, the X mark overlaying the spec- tacles icons under the Project Items column and the keyword Hidden under the Display column indicate that some grid rows and columns are defined but not displayed. More precisely, the fifth scenario and the second response are hidden in Figure 9.17, as evidenced by the absence of corresponding responses in the grid (hidden scenarios are not run). The modeler may hide and redisplay any scenario, control, or response by clicking the corresponding spectacles icon. An examination of the displayed responses confirms the expected behavior.

Figure 9.17 A Process Analyzer window with a populated grid.

Output Analysis 193