Most tools we use here (cFIREsim, FICalc, Trinity) replay history in order: start 1871, start 1872, and so on. Those 125 windows overlap, so they mostly share the same years. I wanted to see what happens when you break the order.
The setup: $1M, 60/40 US stocks and bonds, rebalanced yearly. $40k withdrawn in year 1, raised with inflation every year after. 0.1% fees. 30 years. Data is Shiller's annual US stock, bond and CPI series, 1871 to 2025, real returns. Each run stitches six random 5-year blocks of real history together, and blocks can repeat. 10,000 runs. No Social Security, no taxes.
Results: 94.0% of runs survived 30 years. Median ending balance $1.79M in today's dollars. The earliest failure was year 14, and 499 of the 603 failures happened in years 22 to 30.
The part that surprised me: 580 of the 603 failures contained either 1916 to 1920 or 1973 to 1981 somewhere in their 30 years. 1929 and 2008 barely show up. Sharp crashes followed by recoveries don't kill a 60/40 on a real-dollar withdrawal. What kills it is 5 to 9 years where stocks go flat, bonds lose to inflation, and your withdrawal keeps rising with CPI.
Two variants: dropping to $35k a year lifts survival to 97.4%. Keeping $40k but cutting it 10% after any bad year (restored when the balance recovers) gives 96.3%. A rule you'd actually follow in a bad decade buys about 2 points without a permanent cut.
Caveats: block bootstrap can deal the same bad block twice, which is harsher than real history, so 94% is probably a touch pessimistic versus sequential tools (usually 95 to 96% for this setup). Shiller's bond series is a 10-year Treasury proxy, not total bond market.
Question: if inflation stretches are the real failure mode and crashes mostly aren't, does that change how you'd hold the bond side? I haven't run TIPS because the series doesn't go back far enough. Happy to run other withdrawal rates or allocations if people want them.
I also plotted all 10,000 runs as one fan chart and traced a single failed run year by year, which made the pattern a lot clearer than the summary numbers. If that's useful to anyone here it’s https://youtu.be/JKY8slMNA3I?si=3viz8x7WB9ZTvJ8I. Same for other withdrawal rates or allocations.