Page 730 - ExtendSim User Guide
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Math and Statistical Distributions
Probability distributions
The first type of generator is specified by default in the Run > Simulation Setup >Random Numbers tab and is highly recommended for building models. The second generator is used mainly for backwards compatibility, so that models developed before ExtendSim 4.0 will retain results that are consistent with those older versions of ExtendSim.
Random seeds
A random number stream is a sequence of random numbers; the numbers in the stream are derived based on seed values. The pseudo random number generator is the internal mechanism in ExtendSim which calculates the numbers in the stream.
ExtendSim provides independent random number streams with the ability to specify a seed so that sequences of random numbers can be repeated. A random number is generated based on a seed.
You can specify a seed for the model as a whole in the Run > Simulation Setup > Random Numbers tab. A seed value of 0 or blank uses a random seed; any other value causes repeatable sequences of pseudo-random numbers. This gives repeatable results, allowing you to deter- mine exactly how changes affect the model.
ExtendSim automatically assigns a separate seed to each block in the model that generates ran- dom numbers. To specify your own seed, enter a value in the Use block seed field in the dia- log of those blocks. Any number entered as a seed in a block dialog will result in an independent random number stream that will not change across runs. Since each stream of ran- dom numbers is based on a seed, and you can have a separate seed for each block that gener- ates random numbers, random number generation in ExtendSim is independent. The blocks that generate random arrivals or numbers are the Create and Shutdown blocks (Item library) and the Random Number block (Value library).
Each cell in an ExtendSim database can have an independent seed for the selected distribution. The database seed is dependent on what is set in the Run > Simulation Setup > Random Num- bers tab. The seed field in the Sensitivity Setup dialog is independent of the seed that is set in the Random Numbers tab.
Resetting random numbers for consecutive runs
The Run > Simulation Setup > Random Numbers tab has three options for when a model is run repeatedly:
• Reset random numbers for every run
• Continue random number sequence
• Use Database table __Seed for values
These options are described in “Random Numbers tab” on page 597.
Probability distributions
In real life you cannot know exactly when an event is going to occur until it happens. For example, you do not know when the next customer will enter your store. However, by using the correct statistical distribution you can approximate what happens in the real world.
A distribution (also known as a probability distribution or a random distribution) is a set of random values that specifies the relative frequency with which an event occurs or is likely to occur. ExtendSim’s random number distributions express both a probability that something will occur and a range of values that specify the maximum and minimum value of occurrence.
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