Ronny Kramer Marko Modsching Klaus ten Hagen Ulrike Gretzel Web-based Recommenders for Personalized City Tours: A Usage Analysis. 53-64 2007 ENTER http://dx.doi.org/10.1007/978-3-211-69566-1_6 conf/enter/2007 db/conf/enter/enter2007.html#KramerMHG07