Fundamental reappraisal of metrology, beyond superficial analogies with traditional measurement instruments, is needed if ordinal properties – such as ‘counted fractions’ (bounded by zero and one), performance metrics for ability tests, customer satisfaction, etc - are to be included in an extended quantity calculus for a new SI. Regarding a human being (or other ‘probe’) as a Measurement Instrument with Rasch measurement theory (i.e., postulating a Generalised Linear Model link function z = θ – δ, of a ‘probe’ attribute θ and a ‘target’ attribute δ) not only handles ordinal properties of the measurement system response, Psuccess, but also attribute separability in restitution; essential to underpinning measurement traceability and uncertainty. Examples from person-centred care (e.g., of Alzheimer ’s disease patients) will demonstrate causal Rasch models relating task difficulty and patient ability to explanatory variables such as test sequence entropy and brain atrophy, thereby enabling novel certified reference materials for traceability.