def standard_error(sample_size, successes):
    """
    Calculates the standard error of a sample proportion.
    
    Formula: σp = sqrt [ p(1 - p) / n ]. 
    with:
    p = proportion of successes in sample (successes / sample size)    
    
    :param sample_size: the size of the sample 
    :param successes: the number of successes on the given sample. 
    :return: the standard error on the sample proportion -> σp
    """
    p = successes / sample_size
    return (p * (1 - p) / sample_size) ** 0.5