Random and Exogenous Variation in the Model

FormulaDistribution

QSIM provides a general function writing capability with the FormulaDistribution selection, shown in the list box in Figure 7.1. After you select it and then click the Parameters button, a Formula Manager window (as shown in Figure 5.1) is displayed. From this window, you can write a function that returns a number that is used as the sample. For example, suppose you wanted a mixture distribution with density function

where is exponential with parameter , and is uniform. Figure 7.6 shows a portion of the four windows needed to express this.

Figure 7.6: An Example of a Mixture Distribution

The bottom window is the FormulaDistribution Formula window, and it contains the conditional element . Editing this element provides you with the capability to specify the condition . If the condition evaluates to TRUE, it returns the formula in the True Expression window , which is the appropriate exponential random variable; if it evaluates to FALSE, it returns the formula in the False Expression window , which is the uniform random variable.