A Note About Repeatability of Results Under Simulation

Results obtained through repeated simulations of the same diagram will exhibit some variability because they depend on the randomly generated numbers that are selected for any given simulation run. The use of a seed in simulation forces the software to use the same sequence of random numbers in each simulation, resulting in repeatability. However, you need to be aware that when you use a seed, the same stream of random numbers is utilized, but the order of the utilizations of the stream may differ. This is dependent on the way the diagram was constructed and, for event analysis flowcharts, the order in which event analysis functions and static functions were created. The random numbers are assigned based on an internal ID number assigned to each block or resource when it was created. For blocks, you can display these internal IDs in the diagram by choosing a display format for the Block ID option on the Background and Grid page of the Diagram Style Window.

For example, two people may have projects containing identical diagrams using blocks A and B. If in one diagram, A was created before B, A will take the first random value from the stream and B will take the second. If the other person (in the other project) created the same diagram but created B before A, B will take the first random value from the stream and A will take the second, thus yielding potentially different answers, even though both analysts may have used the same seed in simulation. Note that as the number of simulations increases, this difference will be less significant.