▁which ▁means ▁that ▁no ▁new ▁information ▁is ▁contained ▁in ▁the ▁third ▁dimension ▁of ▁an ▁interval ▁type - 2 ▁f uz zy ▁set . ▁So , ▁for ▁such ▁a ▁set , ▁the ▁third ▁dimension ▁is ▁ignored , ▁and ▁only ▁the ▁FO U ▁is ▁used ▁to ▁describe ▁it . ▁It ▁is ▁for ▁this ▁reason ▁that ▁an ▁interval ▁type - 2 ▁f uz zy ▁set ▁is ▁sometimes ▁called ▁a ▁first - order ▁uncertainty ▁f uz zy ▁set ▁model , ▁whereas ▁a ▁general ▁type - 2 ▁f uz zy ▁set ▁( with ▁its ▁useful ▁third - dim ension ) ▁is ▁sometimes ▁referred ▁to ▁as ▁a ▁second - order ▁uncertainty ▁f uz zy ▁set ▁model . ▁ ▁The ▁FO U ▁represents ▁the ▁bl urr ing ▁of ▁a ▁type - 1 ▁membership ▁function , ▁and ▁is ▁completely ▁described ▁by ▁its ▁two ▁bound ing ▁functions ▁( Fig . ▁ 2 ), ▁a ▁lower ▁membership ▁function ▁( LM F ) ▁and ▁an ▁upper ▁membership ▁function ▁( UM F ), ▁both ▁of ▁which ▁are ▁type - 1 ▁f uz zy ▁sets ! ▁Con sequently , ▁it ▁is ▁possible ▁to ▁use ▁type - 1 ▁f uz zy ▁set ▁mathematics ▁to ▁character ize ▁and ▁work ▁with ▁interval ▁type - 2 ▁f uz zy ▁sets . ▁This ▁means ▁that ▁engine ers ▁and ▁scient ists ▁who ▁already ▁know ▁type - 1 ▁f uz zy ▁sets ▁will ▁not ▁have ▁to ▁invest ▁a ▁lot ▁of ▁time ▁learning ▁about ▁general ▁type - 2 ▁f uz zy ▁set ▁mathematics ▁in ▁order ▁to ▁understand ▁and ▁use ▁interval ▁type - 2 ▁f