Sciencedirect.com http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TF4-4XFPR28-1&_user=3179229&_coverDate=01%2F04%2F2010&_alid=1296986363&_rdoc=1&_fmt=high&_orig=search&_cdi=5216&_sort=r&_docanchor=&view=c&_ct=68707&_acct=C000059970&_version=1&_urlVersion=0&_userid=3179229&md5=885f0886a653acd7067615fcabc34502