CDF is short for cumulative distribution function. CDF measures the probability that a random variable (X) takes on a value either less than or the equivalent of X. When it comes to mining rounds, CDF is used to determine the expected length of a mining round considering the pool’s hashrate and the existing level of network difficulty. CDF is used to determine this because hashrate (and occasionally difficulty) are not absolutely consistent throughout a round of mining, so the anticipated duration is not entirely consistent either.