Morteza Analoui Behrouz Minaei-Bidgoli Mohammad Hossein Rezvani Hierarchical two-tier ensemble learning: a new paradigm for network intrusion detection. 33-40 2007 conf/cikm/2007pikm PIKM http://doi.acm.org/10.1145/1316874.1316881 db/conf/cikm/pikm2007.html#AnalouiMR07