The shapes
Rules of thumb
- Can the value be negative? If not (costs, counts, times), use the right-skewed shape.
- Is it a percentage? Use a percentage shape so it cannot go below 0% or above 100%.
- Is your range lopsided? Use an uneven shape rather than forcing a symmetric one.
- Is it really a few distinct cases, such as a contract won or lost? Use “a few outcomes” rather than a smooth range.
- Do not choose a shape because it gives a nicer answer. Every choice is recorded in the model version.
Fit warnings
The app checks the fit and tells you when it is poor.“Your low, middle and high values do not fit the available shapes exactly”
“Your low, middle and high values do not fit the available shapes exactly”
Your range is lopsided. The app shows the range it would actually use and recommends the uneven-range shape, which matches all three numbers exactly.
“This shape puts about X% of draws below zero”
“This shape puts about X% of draws below zero”
You chose a symmetric shape for something that cannot be negative, such as a cost. Switch to the right-skewed shape, or set a lower limit of zero.
The shape matters most for estimates at the top of the sensitivity ranking. For estimates that barely move the answer, the automatic choice is almost always good enough.