Random and Exogenous Variation in the Model |
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 7.2 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 Seeds...
from the pull-down menus (see Figure 7.3)
on the Simulation window.
Figure 7.3: The Options Pull-down Menu
In the Seed Preferences window, shown in Figure 7.4,
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 7.4: 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.
Copyright © 2008 by SAS Institute Inc., Cary, NC, USA. All rights reserved.