Guides
Overfitting, out-of-sample testing, walk-forward validation, Sharpe ratio, trading costs — the fundamentals, explained plainly, with no product pitch until the end.
Almost every trader has built a strategy with a gorgeous equity curve that quietly bled money the moment it went live. The curve wasn't a lie exactly — it just described the past far better than it described the future. That gap has a name: overfitting.
Read the guide →You backtested it for months. Sharpe over 2, a clean equity curve, drawdowns you could live with. You went live with real money — and it bled. Not a black-swan crash, just a slow, steady loss that never matched what the backtest promised.
Read the guide →Sharpe ratio is the number every trader reaches for first, and the number most often used to make a mediocre strategy look impressive. It's a genuinely useful statistic — but only when you know what it's quietly assuming.
Read the guide →A Sharpe ratio of 3 sounds like a great strategy. It can also be exactly what you'd expect from pure noise, if you tested a thousand parameter combinations and kept only the best one. The deflated Sharpe ratio is the statistic built to tell the two apart.
Read the guide →Both numbers try to answer "how much return did I get for the risk I took" — but they disagree on what counts as risk. That disagreement is the whole difference, and it matters more than it looks.
Read the guide →The formula itself is short. Getting a number you can actually trust — and that means the same thing as someone else's Sharpe ratio — takes a bit more care than plugging numbers into it.
Read the guide →If you only ever check one number before trading a strategy, make it this one. Out-of-sample testing is the closest thing backtesting has to an honesty check — it's the only standard technique that actually simulates not knowing the future.
Read the guide →A single out-of-sample test asks whether your strategy works once, on one holdout period. Walk-forward backtesting asks a harder question: does it keep working, over and over, as you slide through time?
Read the guide →A strategy that won 8 of its last 12 trades sounds like a 67% win rate. It might just as easily be a coin flip that happened to land heads a few extra times — and with only 12 trades, there's no real way to tell the difference.
Read the guide →Commission, slippage, and borrow fees rarely look dangerous on their own — a few basis points here, a fraction of a cent there. But charged on every single trade, over a year of active trading, they add up fast.
Read the guide →An honest backtest can still fail live for a reason that has nothing to do with markets: your live or paper-trading code doesn't actually implement the same logic your backtest tested.
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