To cite eco in publications use:

  Kosuke Imai, Ying Lu, and Aaron Strauss (2011). eco: R Package for
  Ecological Inference in 2 x 2 Tables. Journal of Statistical
  Software, Vol. 42, No. 5, pp. 1-23.
  https://doi.org/10.18637/jss.v042.i05

A BibTeX entry for LaTeX users is

  @Article{,
    title = {eco: R Package for Ecological Inference in 2 x 2 Tables},
    author = {Kosuke Imai and Ying Lu and Aaron Strauss},
    journal = {Journal of Statistical Software},
    year = {2011},
    volume = {42},
    number = {5},
    pages = {1--23},
    url = {https://doi.org/10.18637/jss.v042.i05},
  }