Applies to:
@RISK 5.x and Up

 

How @RISK Computes Parameters for Alt Distributions

• Alt distributions (e.g., RiskPertAlt, RiskTriangAlt) are specified by percentiles instead of standard parameters.

• @RISK finds standard distribution parameters using successive approximations, except for RiskUniformAlt, which has a closed-form solution.

• RiskTrigen works similarly, specifying two triangle parameters as percentiles.

Viewing Standard Parameters

1. In the Define Distribution dialog, click the drop-down next to Parameters: Alternate.

2. Clear the Alternate Parameters checkbox and click OK.

3. The dialog now shows the equivalent standard parameters, e.g.,

   =RiskBetaGeneral(α1, α2, Min, Max)
   

4. Clicking OK replaces the Alt function with the standard one; Cancel keeps the Alt distribution.

Converting Alt Functions via Worksheet

• For some distributions, use RiskTheo statistic functions (e.g., RiskTheoMin, RiskTheoMean) to find standard parameters.

• If formulas exist (e.g., BetaGeneral mean/variance), solve for the parameters algebraically:

  μ = min + α1*(max−min)/(α1+α2)
  σ² = α1*α2*(max−min)² / [(α1+α2)²*(α1+α2+1)]
  

• Some distributions have no closed-form solution—use the Define Distribution dialog instead.

RiskUniformAlt Formula

For RiskUniformAlt(C1,x1,C2,x2), min and max are found algebraically:

min = (x1*C2 − x2*C1)/(C2 − C1)
max = min + (x2 − x1)/(C2 − C1)

• This works because the uniform CDF is linear, so two percentiles uniquely determine min and max.

 

Attachments:
KB1644_StandardParamsFromAltDistribs.xlsm