When does Bitcoin difficulty change?
Bitcoin difficulty changes every 2016 blocks. It adjusts difficulty so that the previous 2016 blocks, if they had been mined at this difficulty, would have taken exactly two weeks to mine. Typically, difficulty increases, which means the previous 2016 blocks were mined faster than two weeks.
It is a percentage adjustment, not an adjustment based on the actual block hashes and which ones would be unqualified of difficulty given a particular adjustment. If the previous 2016 blocks were mined in 12.6 days (10% faster than the 14 days they should take), then difficulty will be increased by 10%, even if more or less than 10% of the previous 2016 blocks would be unqualified under the new difficulty.
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