A start or end price is never a single sale. The end price is the median daily price over the 15 days up to the sneaker's last sale; the start price is the median within 15 days either side of the date one quarter (92 days) earlier.
A sneaker is counted as unchanged (a ratio of 1) when either window is too thin to trust: fewer than 5 sales or 3 trading days, a median implausibly far from the sneaker's own typical price, or no sale in the last 14 days of data. Where asks exist, individual size sales more than 20% below the typical lowest new-condition ask are removed from both windows, since they are usually used or damaged pairs.
Where the retail price is known, a sneaker is not listed as a mover if its start or end price is below half of retail. Prices that far under retail usually reflect used pairs or a mix of cheap sizes rather than the market for new pairs. Such a sneaker still counts toward the index.