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Scenario analysis assigns probabilities to coherent states of the world rather than shocking one variable at a time. Weighting the base case heavily and splitting the remainder between best and worst gives an expected outcome and, more usefully, a dispersion measure for comparing projects of different sizes.
Expected outcome
EV = Best x w + Base x Base weight + Worst x w, where w = (1 - Base weight) / 2
Coefficient of variation
CV % = Standard deviation / |Expected outcome| x 100
Because standard deviation scales with project size. CV normalises it so a small risky project can be compared with a large stable one.
Not necessarily, but splitting the residual weight evenly is a defensible default. Skew it if the downside is genuinely more likely.