Bounded shapes

Triangle, rounded peak (PERT) and even spread: distributions for hard limits.

Use a bounded shape when you can name a lowest possible and a highest possible value. Nothing outside those limits can occur in the model.

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The three shapes

Shape	Looks like	Good for
Triangle	A straight rise to the most common value, then a straight fall	Lowest, most likely and highest, with no other information. Gives the ends a fair amount of weight
Rounded peak (PERT)	A smooth hump around the most common value, with thinner ends than the triangle	A most common value you are fairly sure of
Even spread	Flat: every value between the limits is equally likely	When you have no idea which value is most common. The most common value is not used

The triangle is chosen automatically for hard limits. PERT and even spread are never chosen automatically; select them yourself.

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Choosing between them

1. Do you truly know the most common value? If not, use even spread.
2. Are you fairly sure of it, and do extreme values seem unlikely? Use PERT.
3. Otherwise keep the triangle.

The shape chooser shows each curve, its average and its range side by side. Compare them before choosing.

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When not to use them

If your low and high are a likely range rather than true limits, use a skewed or percentage shape instead. Real limits are rarer than people think: a cost can almost always exceed your highest guess.

Next: Skewed and percentage shapes