Why percentile beats percent-off
A percentile against a product's own history is a defensible measure of discount depth; percent-off a reference price is not.
Percent-off depends entirely on the reference price, which the seller chooses. Percentile depends on what the product has actually cost, which it does not.
The correct computation is time-weighted. A price that held for sixty days should count sixty days, not one row — otherwise a single clearance weekend outweighs a quarter of stability. And ties need midrank, or a product that has never moved reads as its own worst price rather than as flat.
This is also why history cannot be bought. It only accumulates forward, which makes it the one component of a price product that a competitor cannot acquire on the day they decide to compete.
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