1. AltaVista, Available http://www.altavista.com
2. Amazon, Available http://www.amazon.com
3. Searching the Web;Arasu;ACM Transactions on Internet Technology,2001
4. Item response theory: Parameter estimation techniques;Baker,1992
5. Baker, F. B. (2001). The basics of item response theory. In ERIC clearinghouse on assessment and evaluation. College Park, MD: University of Maryland. Available http://ericae.net/irt/baker