import math


def plating_efficiency_multiplier(z):
    """ Plating efficiency correction multiplier for number of mutations per culture
    from [Steward, 1990].

    z is the probability that any given mutant cell will generate a colony.

    :param z: fraction of a culture plated (plating efficiency); 0 < z <= 1
    :return: correction multiplier for number of mutations per culture
    """
    assert 0 < z <= 1
    if z == 1:
        return 1  # by l'Hôpital's rule
    return (z - 1) / (z * math.log(z))