Liam Feldman Carl J. Mueller Dan E. Tamir Oleg V. Komogortsev Usability testing with total-effort metrics. 426-429 2009 ESEM http://doi.acm.org/10.1145/1671248.1671299 conf/esem/2009 db/conf/esem/esem2009.html#FeldmanMTK09