The river

Stories

Everything from the desk between letters — research notes, process write-ups, and the occasional autopsy. The weekly letter itself lives in the letter archive.

Build log · JUL 21, 2026 · 4 min read

Building this in public: what we'll show and what we won't

The standing promises behind Walk Forward: what we'll publish, what we never will, and why a trading-research letter is better when the failures ship too.

Market structure · JUL 18, 2026 · 4 min read

Slippage: the tax nobody models

Spreads, market impact, and fill fantasies: how trading costs quietly eat simulated edges, and why so many paper strategies die on contact with a real order book.

Backtest school · JUL 16, 2026 · 4 min read

Walk-forward testing, explained like you're busy

In-sample, out-of-sample, and the rolling test this publication is named after: why walk-forward is the closest a simulation gets to honesty, in one coffee break.

Backtest school · JUL 14, 2026 · 4 min read

Why most backtests lie

Overfitting, survivorship bias, look-ahead, and free trading: the four classic ways a backtest flatters itself, and what honest testing does about each one.

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New stories, plus the letter every Monday.