Assistance State as a Condition of Functional Measurement: Distinguishing Assisted Performance from Performance at a Lower Assistance State

A functional score is usually read as a property of the person measured. Increasingly it is produced by the person and an assistive environment together: a cane, a caregiver, a cueing system, a rehabilitation robot, or an artificial-intelligence system may supply part of it. The score is real. It answers a different question from the same score without the assistance.The distinction is not new. The International Classification of Functioning, Disability and Health separates capacity from performance in the current environment, with and without assistance. Motor learning has known for forty years that guidance raises performance during practice and lowers it on the unguided retention test. Rehabilitation separates the orthotic effect of a device from its therapeutic effect. Human factors describes the operator who performs better with automation and worse when it fails. These literatures rarely cite one another, and the rule has not been stated as a general principle across them.This paper states it. Assistance state is a condition of functional measurement, as supplemental oxygen is a condition of a six-minute walk. A score under one assistance state cannot be read as a change in what the person supplies when compared with a score under another, unless the change in assistance is what is studied. Where assistance is meant to leave the person more capable, the evaluation includes a prespecified assessment at a lower assistance state, a handback, under a criterion that does not move when the assistance does. Assistance state travels with the observation.