Tornado Sensitivity

Every model has four drivers people argue about and one that actually controls the answer — usually not the same one. The tornado ranks them: hold everything at base, swing one driver to its low and high, measure the output range, sort. Diligence goes to the top bar; the bottom bars stop hijacking meetings.

Required Inputs

• The model — output name, a formula over named drivers (arithmetic + min/max/abs/sqrt/log/exp only), and per-driver low/base/high. The lows and highs should be defensible bounds ("the worst quarter we've seen", "the vendor's contractual ceiling"), not ±10% ritual.
• If the requester has a spreadsheet instead of a formula: extract the output cell's driver chain into a formula first, and show it for confirmation.

Output Format

1. The tornado table — drivers sorted by output swing, with input range, output at each end, and share of total swing. The top driver's share is the headline ("lifetime owns 33% of the uncertainty").
2. The meeting verdict — one paragraph: what deserves diligence, what deserves a decision-and-move-on, and any driver whose bounds are the real problem (huge swing because nobody actually knows the range).
3. The interaction caveat — one-at-a-time ignores correlated drivers; if two move together in reality (price and churn), say so and model the pair as one driver.

Programmatic Helper

Ships scripts/tornado.py — zero dependencies, with a restricted evaluator (driver names + six math functions; anything else is rejected — injection-tested):

python3 scripts/tornado.py run tornado.xlsx --model model.json

Prints base=1.371 · top driver: lifetime (swing 1.097, 33% of total) and writes Summary + Tornado sheets. Requires a code-execution environment.

Quality Checks

• Swings computed by the script, quoted — never reasoned in prose
• Bounds provenance is stated per driver (measured / contractual / guess) — a tornado of guesses is honestly labelled one
• Share-of-swing sums are shown so the ranking's decisiveness is visible
• Correlated drivers are named and the caveat applied to them specifically
• The verdict names what to STOP arguing about — the negative guidance is half the value

Anti-Patterns

• Do not use symmetric ±X% on every driver — uniform ranges produce a tornado shaped by formula structure, not by knowledge
• Do not read the top bar as "most likely to be wrong" — it's "most consequential if wrong"; confidence and consequence are different columns
• Do not run tornado on a model whose formula the owner hasn't confirmed — sensitivity on the wrong model is confidently useless
• Do not let a huge-swing driver with made-up bounds stand — the recommendation there is "go find the real range", not "panic"
• Do not present this as risk analysis — it's attention allocation; downstream probability work still exists