For example, we may choose to sample units with "X" = 1 with a given probability "f", regardless of their frequency in the population (which would necessitate sampling units with "X" = 0 with probability 1 − "f").
For example, we may choose to sample units with "X" = 1 with a given probability "f", regardless of their frequency in the population (which would necessitate sampling units with "X" = 0 with probability 1 − "f").