To cite the loo R package:

Vehtari A, Gelman A and Gabry J (2016). “loo: Efficient leave-one-out cross-validation and WAIC for Bayesian models.” R package version 1.1.0, https://CRAN.R-project.org/package=loo.

To cite the loo paper:

Vehtari A, Gelman A and Gabry J (2016). “Practical Bayesian model evaluation using leave-one-out cross-validation and WAIC.” Statistics and Computing. doi: 10.1007/s11222-016-9696-4.

Corresponding BibTeX entries:

  @Misc{,
    title = {loo: Efficient leave-one-out cross-validation and WAIC for
      Bayesian models},
    author = {Aki Vehtari and Andrew Gelman and Jonah Gabry},
    year = {2016},
    note = {R package version 1.1.0},
    url = {https://CRAN.R-project.org/package=loo},
  }

  @Article{,
    title = {Practical Bayesian model evaluation using leave-one-out
      cross-validation and WAIC},
    author = {Aki Vehtari and Andrew Gelman and Jonah Gabry},
    year = {2016},
    journal = {Statistics and Computing},
    doi = {10.1007/s11222-016-9696-4},
  }