The science of HCI in the third wave is intended to understand user experiences through the filter of the values and contexts of individuals using systems and moreover as filtered through the values and contexts of individual researchers. This is not to neglect the importance of measurement to science and the challenges of measuring user experience (UX). This chapter will discuss how HCI can draw on the methods of modern psychometrics to provide tools for measuring user experiences. In particular, we will introduce bifactor analysis as a way to examine both the conceptual coherence of a questionnaire for measuring UX and also the distinct influences of different facets of the core concept. Further, through looking at modern methods of analysis, in particular treatment of outliers, we also consider how modern statistics are not to be treated as black boxes but require researchers to think more deeply about the people behind the data. Drawing on our work in player experiences, we make the case that psychometrics used well as a tool in UX has an important role to play in HCI as a successor science.