Online toegang naar http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V2W-4MC0TMX-3&_user=457046&_coverDate=04%2F30%2F2007&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000021878&_version=1&_urlVersion=0&_userid=457046&md5=e6869f4206c92c7326f36106e92c4d93
