FIRE glossary
Sensitivity Analysis
Testing how much your FIRE date moves when one input — spend, investment returns, contributions, or current wealth — changes on its own, to see which inputs matter most.
What gets tested
Ember perturbs four inputs one at a time, each by ±10% of its current value: your annual spend, your real investment returns (every asset class's rate is scaled by the same factor, not just a single headline number), your contributions, and your current investable wealth. Each perturbation is run in isolation — spend moves while everything else holds steady, then returns move while everything else holds steady, and so on — so the result for each lever is a clean, one-variable answer to "if only this changed, what would happen to my FIRE date?"
Why the baseline matches your dashboard
Every run — the untouched baseline and all eight perturbations — goes through the same headline calculation that produces the FIRE date shown elsewhere in Ember, overlays and all (mortgage payoff timing, healthcare cost bumps, and similar step-changes recompute for each perturbed scenario rather than being carried over unchanged). That means the sensitivity list's baseline figure is never a simplified stand-in — it's the exact number your dashboard already shows. Results are ranked by the size of the FIRE-age shift, largest first, and the top five are surfaced as your plan's "top sensitivities." If a perturbation would make FIRE unreachable altogether, it's ranked first and shown as "unreachable" — deliberately treated as the most decision-useful result, since a lever that can break the plan matters more than one that shifts it by a few months.
One lever at a time, not the real world
This is a deterministic, single-variable sweep, not a forecast or a probability model. It won't tell you how likely a 10% return shock actually is, what happens when several inputs move together (a recession that cuts both returns and contributions at once, say), or how the order returns arrive in matters — that's what Monte Carlo simulation and sequence-of-returns risk cover instead, and the two are meant to be read side by side, not as substitutes. If your baseline FIRE date is already unreachable, the sweep doesn't run at all, since there's no reachable date for a perturbation to move.
Across borders
None of the four perturbed inputs is currency or tax residency, so a cross-border plan's biggest real-world risks — an FX swing between the currency you earn in and the one you spend in, or a tax-residency move changing the gross-up on withdrawals — aren't directly isolated by this sweep. A "real return −10%" result for a foreign-currency portfolio is really bundling together market performance and currency risk into one number, so it's worth reading alongside the tax-drag-on-withdrawals term and /countries rather than treating it as a pure investment-return test.
Common questions
Why is every test exactly ±10% — is that how much these things actually tend to move?
No — 10% is a fixed, arbitrary yardstick used purely to rank your inputs against each other on a level footing, not a prediction of how much your actual spend, returns, contributions, or wealth are likely to move. Use it to see which lever matters most, not as a real-world range.
What does "unreachable" mean on the sensitivity list?
It means that specific ±10% change, applied on its own with everything else held at your current plan, would push your FIRE date past a reachable horizon. It's ranked at the top of the list on purpose — a lever that can break the plan outright is treated as more decision-useful than one that only nudges the date.
Does this replace Monte Carlo simulation?
No. Sensitivity analysis moves one input at a time in a straight line to show which lever matters most; Monte Carlo simulation randomises returns across many simulated paths to show how likely your plan is to survive at all. They answer different questions and are designed to be read together.
Related terms
See sensitivity analysis in your numbers
The free calculator gives a rough estimate; the full planner models your actual accounts, pensions, residency moves and taxes — with the maths behind every figure shown.