Monte Carlo simulation

Rather than assume one fixed rate of return, FEAsible plays your plan forward through thousands of different randomized futures — each a distinct sequence of monthly market returns drawn from your assumptions. Your plan success is simply the share of those futures in which the money lasts.

So a run of 1,000 paths × 420 months × 6 assets means roughly 2.5 million individual asset-return figures were generated and rolled up — that's the 'simulated asset-returns' number on the results page. More paths means a steadier, less luck-dependent estimate.

Antithetic pairs (a variance-reduction trick)

When enabled, the simulation generates random futures in mirrored pairs: for every randomly drawn sequence, it also runs its mirror image (the good-luck draw paired with its bad-luck opposite). Averaging the pair cancels out a lot of plain luck, so you get a more accurate answer from the same number of paths. It is purely a numerical efficiency technique — it does not change your plan or its assumptions.

Seed, engine version & config fingerprint

• Seed: the starting point for the random-number generator. The same seed with the same inputs reproduces the exact same run, every time — results are deterministic, not a different roll of the dice each visit.
• Config fingerprint: a short code (a hash) computed from all of your inputs. If two runs show the same fingerprint, they were run on identical inputs — handy for confirming two scenarios are truly comparable.
• Engine version: which release of the underlying calculation engine produced the result, so a number can always be traced to the exact logic that made it.

How the optimizers search

An optimizer doesn't guess — it enumerates and scores concrete strategies. For Roth conversions it builds a menu of natural boundaries each year (the top of a tax bracket, or just under a Medicare premium tier), because the best conversion almost always lands exactly on one of those edges. It then assembles many full multi-year candidate schedules from that menu, tries them under different orders of which account/owner to convert first, projects each one, and ranks them by lifetime tax.

So "evaluated 864 candidate schedules across 3 owner orderings and 25 convertible years" means it scored 864 complete conversion plans — that thoroughness is why the recommendation beats hand-tuning. The winner is what you lock in as a strategy layer.

Sustainable-spending solver

The 'sustainable spending' figure is the highest annual spending that still holds your success target (90%). FEAsible finds it by bisection: guess a spending level, run the simulation, then halve the search range up or down based on whether it passed — repeating until it converges. 'Solved in 5 passes' means five such guess-and-check rounds pinned it down.