SIMSCRIPT II.5 Programming Language

Discrete Simulation Concepts
When a sampling distribution cannot be characterized by one of the statistical sampling functions,
declarations can be given that define table look-up sampling variables. A table look-up sampling
variable has a list of possible numeric values together with their associated probabilities. It selects
a sample value by generating a random number and matching it against the possible probability values.
Table look-up variables, hereafter called random variables, are declared in statements of the
form:
or
the system has a name random step variable
every entity has a name random linear variable
Such random variables must be declared as attributes, either of some entity or of the system.
The first form states that sampling is done from a real- or integer-valued sampling distribution
in a steplike manner. The second states that sampling is performed with linear interpolation done
between real sample values. The following illustrations describe how this is done.
Assume that a random variable, or attribute, has the sampling distribution in table 5-6 associated
with it. Note that the cumulative probabilities in the left-hand column range from 0.0 to 1.0. Sampling
is performed by generating a probability value using random.f(1), matching it with a value
in column 1, and selecting an appropriate value from column 2. Since samples from random.f are
always between 0.0 and 1.0, and are uniformly distributed between these extremes, the samples
drawn from column 2 will be chosen randomly.
Table 5-6. Sampling Distribution (Example)
Cummulative Probability
0.00
0.10
0.20
0.25
0.38
0.45
0.60
0.77
0.90
0.99
1.00
Sample Value
0.0
1.0
2.5
3.0
9.0
11.8
20.9
30.0
33.3
50.0
66.7
219