To cite psychotree in publications use the references for the relevant functions/data sets:

If you use Bradley-Terry trees, bttree(), please cite:

Carolin Strobl, Florian Wickelmaier, Achim Zeileis (2011). Accounting for Individual Differences in Bradley-Terry Models by Means of Recursive Partitioning. Journal of Educational and Behavioral Statistics, 36(2), 135-153. doi:10.3102/1076998609359791

If you use Rasch trees, raschtree(), please cite:

Carolin Strobl, Julia Kopf, Achim Zeileis (2014). Rasch Trees: A New Method for Detecting Differential Item Functioning in the Rasch Model. Psychometrika, forthcoming. doi:10.1007/s11336-013-9388-3

If you use rating scale or partial credit trees, rstree()/pctree(), please cite:

Basil Abou El-Komboz, Achim Zeileis, Carolin Strobl (2014). Detecting Differential Item and Step Functioning with Rating Scale and Partial Credit Trees. Technical Report 152. Department of Statistics, Ludwig-Maximilians-Universitaet Muenchen. URL http://epub.ub.uni-muenchen.de/17984/

If you use the SPIEGEL Studentenpisa data subsample, SPISA, please cite:

Sabine Trepte, Markus Verbeet (2010). Allgemeinbildung in Deutschland -- Erkenntnisse aus dem SPIEGEL Studentenpisa-Test. VS Verlag, Wiesbaden.

Corresponding BibTeX entries:

  @Article{,
    title = {Accounting for Individual Differences in {B}radley-{T}erry
      Models by Means of Recursive Partitioning},
    author = {Carolin Strobl and Florian Wickelmaier and Achim
      Zeileis},
    journal = {Journal of Educational and Behavioral Statistics},
    year = {2011},
    volume = {36},
    number = {2},
    pages = {135-153},
    doi = {10.3102/1076998609359791},
  }
  @Article{,
    title = {Rasch Trees: A New Method for Detecting Differential Item
      Functioning in the {R}asch Model},
    author = {Carolin Strobl and Julia Kopf and Achim Zeileis},
    journal = {Psychometrika},
    year = {2014},
    note = {Forthcoming},
    doi = {10.1007/s11336-013-9388-3},
  }
  @TechReport{,
    title = {Detecting Differential Item and Step Functioning with
      Rating Scale and Partial Credit Trees},
    author = {Basil {Abou El-Komboz} and Achim Zeileis and Carolin
      Strobl},
    institution = {Department of Statistics,
      Ludwig-Maximilians-Universit\"at M\"unchen},
    year = {2014},
    type = {Technical Report},
    number = {152},
    month = {January},
    url = {http://epub.ub.uni-muenchen.de/17984/},
  }
  @Book{,
    title = {{A}llgemeinbildung in {D}eutschland -- Erkenntnisse aus
      dem {SPIEGEL} {S}tudentenpisa-{T}est},
    editor = {Sabine Trepte and Markus Verbeet},
    year = {2010},
    publisher = {VS Verlag},
    address = {Wiesbaden},
  }