Block bootstrap
Building possible futures by copying one-year stretches of real history, in a different, random order.
It's how Look ahead imagines futures without making up data: it takes one-year pieces of the asset's real history and sticks them together in a different, random order, thousands of times.
In the chart, the green line is what happened to the S&P 500 over the last 20 years. The grey ones are three futures built from one-year blocks picked at random from its whole history since 1926: they have real bad years and real good years, but in a different order.
That way the risk, with its drops and streaks, comes from what actually happened, without assuming history will repeat itself exactly.
It's a moving block bootstrap: a random starting point is picked in the series of monthly returns and 12 consecutive months are copied; this repeats until the time span is filled. If a block runs past the end of the series, it continues from the beginning.
Copying blocks, instead of single months, keeps streaks: in markets, bad months come together. With single months, long drops would barely show up.
Compass keeps risk and growth apart: the shape of returns comes from history and the growth rate is an assumption you choose. Each simulation builds 2,000 paths.
- What happened
- Futures built from blocks
$100 at the start. 12-month blocks picked at random from all history since 1926.
Try it yourself
Each square is a real year of the S&P 500, from 1927 to 2025: green if it rose, red if it fell. Tap “Build a future” and watch 20 years get built from years picked at random.
Real history, year by year
You start with $100.
- The future being built
- Earlier futures
To make it visible, each block here is a calendar year. In Look ahead the 12-month blocks can start in any month, and each calculation builds 2,000 futures.
Where you see it in Compass
- Simulator · Look aheadIn Look ahead, every possible future is built this way.
- MethodologyIn the methodology, the full detail.
Related terms
Frequently asked questions
Why one-year blocks and not single months?
Because bad months come together. Blocks keep streaks like those of 2008 or 2022; with single months, long drops would almost disappear.
Does this predict the future?
No. It shows a spread of possible futures with the risk history had. The future can differ from everything that happened.
Calculated and reviewed by Compass. Sources, checks and method
Figures with data as of October 26, 2026. Educational content. Not financial advice. Past results don't guarantee future results.