Source: Sixk, 'Pair trading - history length for spread calculation', quant.stackexchange.com/questions/80948, plus the answer by 'THATS MY QUANT' (quant.stackexchange.com/a/80953). The question runs a cointegration z-score pair trade on crypto perps and finds the results swing with the lookback (200 vs 500 candles). The answer gives three rules: (1) do not choose the window by backtest or p-value; (2) measure whether the divergence is large enough to pay fees and slippage; (3) ask why the pair relationship exists and when it breaks down, and stand aside then. This hypothesis puts all three into one design. PAIR AND REASON: LTCUSDT.BYBIT and BCHUSDT.BYBIT linear perps. Both are 2011-2017-vintage PoW payment forks of the Bitcoin codebase. They sit in the same exchange 'legacy PoW' bucket and have a largely overlapping holder and market-maker base, with no DeFi or L2 narrative to drive one apart from the other. The relative price is therefore driven mostly by shared old-coin beta plus temporary flow imbalances. This is not the same-asset cross-venue spread that just died (SOL Bybit/OKX, HYPE spot/perp). That spread is a market-maker speed contest, which the answer explicitly warns against. This is a cross-ASSET relative-value spread with dislocations of 6-10%, not a few bps. WINDOW ROBUSTNESS: the fair-value anchor is an EWMA of log(LTC/BCH) with a half-life tuned over a broad range (60-180 4H bars, 10-30 days). The pre-study shows the sign of the edge holds across that range. No cointegration p-value is used for selection. FEE FLOOR: entry needs an ABSOLUTE log deviation of at least 6%, not just a z-score, so every trade starts about 27x above the 0.22% two-leg round trip. BREAKDOWN VETO: if the 24h volume of one leg relative to its own 30-day median is more than 1.8x the other leg's, the divergence is treated as idiosyncratic news, which is the answer's SAP-earnings example. In that case no entry is taken. Venue Bybit was chosen because it is 2.0% of the corpus, against a quota of 5% or more. LTC and BCH are nearly unused in the corpus, and Bybit 1H bars run from Oct/Dec 2020, about 5.8 years. Bars are resampled to 4H in-strategy, or 4H bars are used directly.