Is the notion of validity valid in HCI practice?

Much attention has been paid in the recent literature to the notions of validity, thoroughness, and effectiveness of different Usability Evaluation Methods (UEMs). Calculation of these makes sense if a study aims to compare UEMs, but not, it is argued here, if a study aims to evaluate a given application. Illustrated by a case study employing different UEMs, it is argued here that for practitioners, UEMs serve to supplement, rather than compete against, each other. The study shows also that it is not possible to calculate validity, thoroughness, and effectiveness in actual usability studies