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3.49. Bimodal or Mixed Distribution

Applies to: @RISK 5.x–7.x

How can I create a bimodal distribution in @RISK? I want a distribution that is a mix of one distribution some percentage of the time, and a different distribution the rest of the time.

Although it's possible to do the whole thing in one cell, it's clearer if you use several "helper cells". That will also make it easier to find what is wrong if the final distribution doesn't behave as you expected.

Please refer to the attached example in conjunction with these steps:

  1. Place the two desired distributions in two cells (B15:B16).

  2. In a third cell (C16), place the proportion of the final mix that should come from the first distribution. The rest will come from the second distribution.

  3. In a fourth cell (B18), place a RiskBernoulli( ) to determine which distribution gets used in that iteration. (RiskBernoulli returns 1 the stated percentage of the time, and 0 the rest of the time.)

  4. Finally (B20), construct an IF(fourth cell, first cell, second cell).  That is the final mixed distribution.

  5. Recommendation: Wrap the final distribution in a RiskMakeInput( ). That way, any sensitivity analysis that you do will treat the final distribution as an input and will not go back to the original inputs or the RiskBernoulli( ). See also: All Articles about RiskMakeInput.

I have a similar requirement, but instead of choosing a distribution probabilistically, I need to use one distribution below a certain x value and a different distribution above that value.

The RiskSplice( ) function is designed for this application. In the @RISK ribbon, click Insert Function and find RiskSplice( ) among the special distributions.

Last edited: 2015-06-19


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