Master Usability Scaling (MUS) is a measurement method for developing a universal usability continuum based on magnitude estimation and master scaling. The universal usability continuum allows true ratio comparisons, potentially between all items measurable by the construct of usability (attributes, tasks, or products -- software or hardware) that have contributed to the meta-set by following the procedures prescribed. This paper describes the background for MUS, data reduction, and cases studies in software usability assessment.MUS is based on a new measurement method of usability, Usability Magnitude Estimation (UME) [9], where users estimate usability magnitude according to an objective definition of usability. UME allows all items measured within a single usability activity to be compared across one continuum. MUS utilizes UME to assess standard reference tasks across different usability activities to construct one meta-set of data. This meta-set of data can be represented as a universal usability continuum. MUS is simple to administer, easy to comprehend, and with advanced underlying calculations, powerful to use. The MUS continuum has the potential to be a widespread, robust, universal measurement scale of usability.