def unskew_S1(S1, M, N):
    """
    Unskew the sensivity indice
    (Jean-Yves Tissot, Clémentine Prieur (2012) "Bias correction for the
    estimation of sensitivity indices based on random balance designs.",
    Reliability Engineering and System Safety, Elsevier, 107, 205-213.
    doi:10.1016/j.ress.2012.06.010)
    """
    lamb = 2 * M / N
    return S1 - lamb / (1 - lamb) * (1 - S1)
