Controlling the Seed Values

Random and Exogenous Variation in the Model

Controlling the Seed Values

The QSIM application gives you considerable control over the streams of random numbers used in simulations. Each instance of a random number has its own unique generator. The control panel for each provides a mechanism for setting the seed and time units associated with the random variable. The exponential control panel in Figure 6.2 shows this. By default, a seed is selected when the component is instantiated. This seed is selected from a seeds dataset, named QSEEDS, in the SASHELP library and selected to guarantee no overlap with any other random variable seed values for 100,000 observations. Moreover, each time you run the QSIM Application, the selection of seeds continues from where it last left off. You can control these by raising the Seed Control panel by selecting the Options $\ra$ Seeds... from the pull-down menus (see Figure 6.3) on the Simulation window.

Figure 6.3: The Options Pull-down Menu

In the Seed Preferences window, shown in Figure 6.4, you can increase the default distance between seeds, and when you click the Reset Seeds button you reset the seeds to the original values that were automatically assigned when the components were instantiated. This enables you to rerun the simulation with the same seed values used initially.

Figure 6.4: The Seed Control Panel

The Reset Seed Stream button resets the seed stream to the beginning of the QSEEDS dataset. This will result in use of the seeds that were used when the application was first installed on your computer. You can also edit the QSEEDS data set in the SASHELP directory to change the seed values although this is not recommended.

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