1-Way Sensitivity Range (Tornado)
Swing one input between its low and high bound and see how far the result moves — the bar length of a tornado diagram.
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The engineering
A one-way sensitivity holds every assumption at its nominal value except one, then swings that single input from its low bound to its high bound and reads off how far the metric of interest (NPV, life-cycle cost, IRR) moves. The magnitude of that swing is the length of one bar in a tornado diagram; sorting the bars longest-first tells you which assumptions actually drive the decision and which are noise.
Rule of thumb: rank your candidates by swing before you spend a dollar refining any single estimate — the widest bar is where better data buys the most confidence. The linear form here assumes the output responds proportionally to the input over its range; if your model is strongly nonlinear, feed the actual output values at low and high instead of a constant per-unit slope.
Watch the sign of the sensitivity: a negative slope means the low input gives the high output (an inverted bar). The downside/upside rows are reported relative to the baseline so an inverted response still reads correctly.
Where this math comes from
Systematic sensitivity analysis for public investment matured inside the U.S. federal budgeting apparatus. OMB Circular A-94, first issued in 1972 and revised in 1992, directed agencies to discount future costs and benefits and — crucially — to test how conclusions hold up when key assumptions vary, rather than reporting a single point estimate as if it were certain.
The tornado diagram itself came out of decision-analysis practice at firms like SRI and Strategic Decisions Group in the 1970s–80s, where analysts needed a compact picture ranking dozens of uncertain variables by their effect on value. Plotting each variable's one-way swing as a horizontal bar and sorting them widest-at-top produces the funnel shape that gave the chart its name.
- 1972U.S. OMBCircular A-94 first issued, requiring benefit-cost analysis with sensitivity testing.
- 1976Ronald Howard / SRI decision analystsFormalize one-way sensitivity ranking in applied decision analysis.
- 1983Strategic Decisions GroupPopularize the tornado diagram for prioritizing uncertain variables.
- 1992U.S. OMBRevises Circular A-94, setting standard discount rates and sensitivity guidance for federal projects.
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