code: def unskew_S1(S1, M, N):
      
      lamb = (2 * M) / N
      return S1 - lamb / (1 - lamb) * (1 - S1)
docs: 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)