How to use Monte Carlo results without over-trusting them
A good portfolio projection should help you ask better questions. It should not make the future look cleaner than it is. This guide explains how to read the simulator and how to compare different portfolio setups.
Start with the downside
Many investors focus first on the median final value because it feels like the main answer. The downside percentiles are often more important. P10 and P25 outcomes help answer practical questions: what happens if returns are weaker, correlations rise, or the timing of withdrawals is unlucky?
- Run the base portfolio using realistic starting value, contribution, tax, and rebalance settings.
- Look at P10, P25, median, and max drawdown before looking at upside values.
- Change one assumption at a time so you can tell what actually improved the plan.
Compare portfolios by range, not just final value
Two portfolios can have similar median outcomes but very different downside ranges. A concentrated portfolio may show a higher P90 outcome but also a wider spread between P10 and P90. A diversified portfolio may have less explosive upside but a more stable downside band.
Use drawdown as a behavior check
Maximum drawdown is a useful stress-test metric because the hardest part of a plan is often staying invested during a decline. A portfolio that looks strong at year 30 may still be difficult to hold if the simulated path includes large peak-to-trough losses.
When comparing strategies, ask whether the expected drawdown is something you could realistically tolerate. If the answer is no, a lower-return portfolio may still be more useful because it is easier to follow.
Understand dividends and withdrawals
Dividend yield, dividend growth, and reinvestment settings can change the shape of a long-term plan. Reinvesting dividends keeps the income inside the portfolio. Taking dividends as cash can support spending, but it may reduce compounding. Withdrawal settings can be modeled as monthly dollar amounts or as an annual percentage of the portfolio.
For retirement-style analysis, test both a fixed dollar withdrawal and a percentage withdrawal. Fixed withdrawals can be more vulnerable to poor early returns, while percentage withdrawals automatically adjust spending as the portfolio changes.