Key takeaways
- Changing one lever at a time gives every difference an author.
- Comparisons only mean something on the same horizon, in the same dollars, with the same definition of done.
- The spread teaches more than the winner — wide swings signal assumption-dependence.
Change one lever at a time
A scenario that changes five things at once produces a difference with no author. The discipline is surgical: clone the baseline, move one variable — return, contribution, retirement age, spending — and let the outputs attribute themselves. Capital Allocation’s driver detection automates exactly this: it re-runs the baseline with each differing variable alone and reports the biggest mover.
Measure with the same ruler
Comparisons only mean something when every scenario shares a horizon, an inflation treatment, and a definition of “done.” Net worth at the same target age, in the same dollars, under the same withdrawal rule — then the differences are real. Deterministic outputs (this site’s kind) make that easy: same inputs, same outputs, every time.
Read the spread, not the winner
The point of best/baseline/worst is not to pick the flattering line — you do not control returns. It is the width of the spread that teaches: a plan whose outcomes swing wildly with one percentage point of return is fragile in a way a contribution-driven plan is not. Prefer plans whose success depends on levers you hold.
Example · the spread is the lesson
Baseline projects $1.05M at the target age. “Return −2%” lands at $780K; “Contribute +$250/mo” lands at $1.21M. The first gap is weather. The second is a decision — and only one of those is yours to make.
Common traps
Comparing a hopeful scenario against a strawman baseline. Letting an aggressive return assumption smuggle in optimism everywhere. Forgetting that a “lower spending” scenario has to be lived, not just modeled. The cure for all three is the same: keep scenarios boring, singular, and honest.
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