Run a what-if analysis

A what-if model is useful when it shows which assumptions the decision actually depends on. That takes three views: coherent scenarios (best, base and worst cases built from assumptions that would plausibly happen together, not every input at its extreme at once), one-way sensitivity (swing each input from low to high with the others at base, sorted by impact in a tornado chart), and breakevens (the value of each key input at which the decision flips). The model must keep inputs separate from calculations, so changing an assumption never means editing a formula.

Build a what-if analysis in excel for this decision:

Uncertain inputs:

1. Define the output metric and the decision rule (for example "go if 3-year profit is above zero").
2. Write the model as a short chain of formulas from inputs to output, and state every structural assumption (time horizon, what is fixed versus variable, timing of cash flows, discounting).
3. Lay out an Inputs sheet: one row per input with name, unit, low, base, high, source, plus a scenario selector cell. Give each input a named range.
4. Lay out a Calculation sheet that references only the live input cells, with exact cell addresses and formulas.
5. Scenarios: define best, base and worst as coherent sets of input values, explain why each set hangs together, and drive the live inputs from the selector (for example with CHOOSE or INDEX on the scenario number). In Excel, also mention Scenario Manager as an option.
6. Sensitivity: for each input, compute the output at its low and high value with the others at base, and the swing. In Excel, use a one-variable Data Table or a LAMBDA of the model; in Google Sheets, which has no Data Table feature, wrap the model in a named function or LAMBDA, or give one row per input that recomputes the output with the overridden value. Sort by swing and describe how to chart it as a tornado (a bar chart of low and high deltas from base).
7. Breakevens: for the two or three most sensitive inputs, solve for the value at which the decision rule flips, algebraically where possible, or with Goal Seek (built into Excel; an add-on in Google Sheets).
8. Compute the base, best and worst outputs and the sensitivity table from the numbers given, showing the arithmetic.

• Use only the user's numbers. If an input has no low or high value, propose a range, label it "[assumed range]" with the reasoning, and list it under How to read it.
• If an important input seems to be missing from the list (for example taxes, ramp-up time or one-off costs), name it and ask whether to add it rather than silently inventing a value.
• Every formula must work in excel as written; use function names and separators for an English-locale setup and say so.
• Present the model as a decision aid, not a recommendation: the decision belongs to the user, and the model is only as good as its ranges.
• Keep it auditable: no hard-coded numbers inside formulas, and no circular references.

Model

The output metric, decision rule, formula chain and structural assumptions.

Inputs sheet

A table: cell | name | unit | low | base | high | source.

Calculation sheet

A table: cell | label | formula.

Scenarios

A table: input | worst | base | best, then the output for each scenario and the selector formula.

Sensitivity

A table sorted by swing: input | output at low | output at high | swing; then the formulas and the tornado chart steps.

Breakevens

Each input's breakeven value and how it was found.

How to read it

Three to five bullets: which assumptions matter most, which ranges were assumed, and what to verify before deciding.