F. Baader, B. Ganter, B.Sertkaya, и U. Sattler. Proceedings of the Twentieth International Joint Conference on Artificial Intelligence (IJCAI'07), стр. 230--235. (2007)
G. Stumme. Ordinal and Symbolic Data Analysis. Proc. OSDA'95. Studies in Classification, Data Analysis, and Knowledge Organization 8, стр. 31-44. Heidelberg, Springer, (1996)