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14.9. Random Number Seed in RISKOptimizer

Applies to: @RISK 6.x/7.x Industrial Edition

Can I use a fixed initial seed for RISKOptimizer, to make an optimization repeatable?

Yes, you can set this in Simulation Settings, on the Sampling tab.

Is there any guidance on using a fixed seed across multiple simulations with RISKOptimizer?

You're asking about this setting in @RISK: Simulation Settings » Sampling » Multiple Simulations. By default it's set to "All Use Same Seed". We recommend that setting, because it makes it easier to interpret the optimization log. For example, let's say B4 and B5 are your adjustable cells. You see in the log that the simulation results are the same for (B4=1, B5=1), (B4=1, B5=2), (B4=1, B5=3), but are different as soon as the value of B4 changes. You can interpret this as a hint that the value of B5 doesn't matter for simulation results. On the other hand, with Multiple Simulations set to "Use Different Seeds", you will probably never see identical simulation results.

Depending on details of the model, the optimization algorithm may make a similar observation that B5 doesn't seem to make a difference to the results, and allocate less time to attempts to improve the results by changing B5.

When the Genetic Algorithm is used — either because you selected it, or because you selected Automatic and RISKOptimizer selected the GA — there's a second reason to select "All Use Same Seed" in Simulation Settings. The GA can sometimes backtrack and reuse the same set of adjustable cells. If this happens, but the seed is different, the simulation results will be different also, and this may make the GA converge more slowly than with "All Use Same Seed".

Last edited: 2015-07-23

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