Applied Mathematical Sciences, Vol. 7, 2013, no. 102, 5071 - 5084 HIKARI Ltd, www.m-hikari.com http://dx.doi.org/10.12988/ams.2013.36320 Interval Type-2 Fuzzy Logic System Model in Measuring the Index Value of Underground Economy in Malaysia Abd. Fatah Wahab Department of Mathematics, Faculty of Science and Technology University Malaysia Terengganu, Malaysia fatah@umt.edu.my Mohammad Izat Emir Zulkifly Department of Mathematics, Faculty of Science and Technology University Malaysia Terengganu, Malaysia emir_zul@yahoo.com Hanafi A. Rahim Department of Mathematics, Faculty of Science and Technology University Malaysia Terengganu, Malaysia hanafi@umt.edu.my Rozaimi Zakaria Department of Mathematics, Faculty of Science and Technology University Malaysia Terengganu, Malaysia rozaimi_z@yahoo.com Copyright ©2013 Abd. Fatah Wahab, Mohammad Izat Emir Zulkifly, Hanafi A. Rahim and Rozaimi Zakaria. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.