def sample_size_z(z, std, max_error):
    """
    Return the sample size required for the specified z-multiplier, standard deviation and maximum error.
    :param z: z-multiplier
    :param std: Standard deviation
    :param max_error: Maximum error
    :return: Required sample size
    """
    return pow(z, 2.0) * pow((std / max_error), 2.0)