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It uses the 3.5 day median price as reported via the witnesses price feeds.

Median over all witnesses, or just the top 20 or top 50?
And by the time of reward, which time's median price feed does it use?

The 20 top witness plus one randomly chosen backup witness will make up the price feed sample for the current round. The median is taken across those 21 witnesses.

I am 95% sure that the price at the time of payout is the median of those medians across the past 3.5 days.

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