def _cliffs_delta(x, y):
    """
    Calculates Cliff's delta.
    """
    delta = 0
    for x_val in x:
        result = 0
        for y_val in y:
            if y_val > x_val:
                result -= 1
            elif x_val > y_val:
                result += 1
        delta += result / len(y)
    if abs(delta) < 10e-16:
        # due to minor rounding errors
        delta = 0
    else:
        delta = delta / len(x)
    return delta