Given n tests and k successes, return efficiency and confidence interval.

def binomialInterval(n, k, alpha = 0.68):
    """
    Given n tests and k successes, return efficiency and confidence interval.
    """
    e = float(k)/n
    delta_e = 1/float(n) * np.sqrt(e * (1 - e) * float(n)) * alpha/0.68
    return e, [e - delta_e, e + delta_e]