HomeTechniques and Tips@RISK DistributionsSum of Distributions Must Equal Fixed Value

3.12. Sum of Distributions Must Equal Fixed Value

Applies to: @RISK 4.x–7.x

I have several continuous distributions that vary independently, but I need them always to add up to a certain value.  Is there any way to accomplish this?

Please see the attached example.

The technique is to let the distributions vary randomly, but have an equal number of helper cells.  Each helper cell is a scaled version of the corresponding distribution.  "A scaled version" means that the first helper cell equals the first actual distribution multiplied by the desired total and divided by the actual total, and similarly for each of the helper cells.  In this way you are guaranteed that the helper cells always add up to the desired value.

Your workbook formulas should all refer to the helper cells, and not to the original distributions.  If you want to record the values of the helper cells during a simulation, you can designate them as @RISK output cells.

Please note that this technique is suitable for continuous distributions. If you need discrete distributions to add up to a fixed total, you can't use this technique because the scaled versions usually won't be whole numbers.

Additional keywords: Total of distributions equals constant, Fixed value for total

Last edited: 2016-12-15

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