Metrological assurance of qualitative evaluations, such as made by a person acting as a Measurement Instrument (e.g. in person-centred care) seems to be possible using a generalised linear model specifically based on the Rasch psychometric approach. Traditional classical test theory and many of the usual tools of statistics cannot work reliably on ordinal or nominal scales typical of person responses, rating questionnaires or other qualitative evaluations. This talk recalls how to establish metrological references (item banks of task difficulty, for example) and uncertainty budgets for categorical data (using informational entropy). New insight will then be given into how the reliability of subjective measurement systems and the Rasch model can be evaluated with respect to measurement scale shift and scale stretching with novel tools analysing rating scores and logistic regression residuals. Examples will range from physical rehabilitation to cognitive assessment.