HomeTechniques and Tips@RISK DistributionsLeft Skewed or Negative Skewed Log-normal Distribution

3.26. Left Skewed or Negative Skewed Log-normal Distribution

Applies to: @RISK 5.0 and newer

I want to create a left skewed (negatively skewed) log-normal distribution function with @RISK, using three percentiles. Using the @RISK Define Distribution window with alt parameters, I put in 0 as my 5th percentile, .018 as my 70th percentile, and .021 as my 95th percentile. But then the Define Distribution window says "Unable to graph distribution", and the function returns a #VALUE error. Can you explain why @RISK won't allow this to be done?

This is based on the definitions for skewness and the domains for mu and sigma in a log-normal (mu > 0 and sigma > 0, μ and σ both positive). The skewness will never be negative if all terms in the expression are always positive.

You do have several workarounds, however:

Additional keywords: RiskBetaGeneral, RiskLognorm

Last edited: 2015-06-19

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