To cite package ‘birtr’ in publications use:

  Kim S (2017). _birtr: The R Package for "The Basics of Item Response
  Theory Using R"_. R package version 1.0.0,
  <https://CRAN.R-project.org/package=birtr>.

A BibTeX entry for LaTeX users is

  @Manual{,
    title = {birtr: The R Package for "The Basics of Item Response Theory Using R"},
    author = {Seock-Ho Kim},
    year = {2017},
    note = {R package version 1.0.0},
    url = {https://CRAN.R-project.org/package=birtr},
  }