Yutao Zhong Xipeng Shen Chen Ding A Hierarchical Model of Reference Affinity. 48-63 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=2958&spage=48 2003 conf/lcpc/2003 LCPC db/conf/lcpc/lcpc2003.html#ZhongSD03