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 6.1. After you select it and then click the Parameters button, a Formula Manager window (as shown in Figure 4.1) 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

f(x)=.5 f₁(x) + .5 f₂(x)

where f₁(x) is exponential with parameter $\lambda$ , and f₂(x) is uniform. Figure 4.1 shows a portion of the four windows needed to express this.

Figure 6.6: 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.

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