André de Korvin C. McKeegan Robert M. Kleyle Knowledge acquisition using rough sets when membership values are fuzzy sets. 237-244 1998 6 Journal of Intelligent and Fuzzy Systems 2 http://iospress.metapress.com/content/uet6g0t9vw6vn7e4/ db/journals/jifs/jifs6.html#KorvinMK98