How often a pattern's factors actually move together in extreme conditions, not just on average.
Schmidt-Stadtmüller 2006: an empirical estimator for whether two factors co-move in their tails (extreme upper or lower decile). λ ≥ 0.25 means the factors historically co-move when the market is stressed — structural pattern, not coincidence. λ < 0.10 means they're independent in tails — the apparent setup is probably random alignment. We use this to dampen pattern confidence when tail-alignment is weak.