Index & ETF InvestingA backtest answers a question every investor secretly wants answered: "If I'd been holding this allocation through the last 20 years, what would actually have happened?" It's not a crystal ball — but done right, it turns a vague hunch into a concrete picture of risk. Here's how to run one and, just as important, how to read it.
A backtest applies your chosen allocation to real historical returns to show how it would have performed — including its worst year and deepest drawdown. It can't predict the future, and a mix that looks perfect in hindsight may be overfit. Used honestly, its greatest value isn't the ending balance; it's revealing how much you'd have had to endure to get there.
What a backtest is
Take an allocation — say 60% U.S. stocks, 25% international, 15% bonds — and "replay" it against real month-by-month market data from the past. The result is the balance path you would have ridden: the growth, yes, but also the drops, the flat years, and the single worst stretch. It's history applied to your specific choices.
Four steps to a useful backtest
- 1. Set a realistic allocation. Use the funds you'd actually hold, in weights that add to 100%. Test the portfolio you own, not an idealized one.
- 2. Choose an honest window. Include at least one full downturn — 2008 or 2022 — not just a bull run. A backtest that starts in 2009 flatters everything.
- 3. Read the worst year first. Before the ending balance, find the deepest 12-month loss. That's the number that tests whether you could have held on.
- 4. Compare against an alternative. A result means little in isolation. Put your allocation beside a benchmark or a different strategy to see the trade-off.
What a backtest can't tell you
Honesty matters here. The future won't repeat the past, so a backtest is a stress test, not a forecast. Beware allocations that look flawless in hindsight — they're often "overfit" to the exact history you tested and fragile in new conditions. And a backtest can't measure the emotional test of living through a drawdown in real time. Treat it as a way to understand risk, not to chase the highest historical number.
Run one without the spreadsheet
You don't need to build this yourself. The Investment Safety Check is a ready-made backtest: set your allocation across everyday index ETFs, pick any window from 2006 to 2025, and watch it compound month by month — with a rules-based model overlaid and the worst year reported for both. Start with the market's worst drawdowns to choose a window worth testing.
From backtest to discipline
A backtest often ends with a realization: the smoother path is worth more than the flashier one. Translating that into a repeatable rule — rather than a one-time insight — is the point of Alpha Edge, a monthly update built to keep drawdowns shallow enough to stick with.