To cite package 'SGP' in publications use:

Damian W. Betebenner, Adam VanIwaarden, Ben Domingue and Yi Shang (2014). SGP: An R Package for the Calculation and Visualization of Student Growth Percentiles & Percentile Growth Trajectories. (R package version 1.2-0.0. URL http://centerforassessment.github.com/SGP/

Corresponding BibTeX entry:

  @Manual{,
    title = {{SGP}: An R Package for the Calculation and Visualization
      of Student Growth Percentiles \& Percentile Growth
      Trajectories.},
    author = {Damian W. Betebenner and Adam {VanIwaarden} and Ben
      Domingue and Yi Shang},
    year = {2014},
    note = {R package version 1.2-0.0},
    url = {http://centerforassessment.github.com/SGP/},
  }