To cite package 'SixSigma' in publications use:

Cano EL, Moguerza JM, Redchuk A (2012). Six Sigma with R, volume 36 series Use R! Springer, New York.

Cano EL, Moguerza JM, Corcoba MP (2015). Quality Control with R, series Use R! Springer, New York.

Corresponding BibTeX entries:

  @Book{,
    title = {Six Sigma with R},
    author = {Emilio L. Cano and Javier M. Moguerza and Andres
      Redchuk},
    year = {2012},
    publisher = {Springer},
    address = {New York},
    volume = {36},
    series = {Use R!},
  }

  @Book{,
    title = {Quality Control with R},
    author = {Emilio L. Cano and Javier M. Moguerza and Mariano Prieto
      Corcoba},
    year = {2015},
    publisher = {Springer},
    address = {New York},
    series = {Use R!},
  }