CDF stands for cumulative distribution function; a function that measures the probability that a random variable X takes on a value less than or equal to x. In the case of mining rounds, the CDF is used to calculate the expected duration of a given mining round given the pool’s hashrate and the current network difficulty. A CDF is used to calculate this because hashrate (and sometimes difficulty) are not perfectly constant throughout the mining round, so the expected duration is not perfectly constant either.

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