VIRGINIA Apgar can be regarded as the founding parent of modern clinimetrics.1 The score she constructed2 more than 40 years ago was a pioneer attempt to convert an intangible clinical phenomenon into a formally specified measurement.
The object of Apgar's attention—the clinical condition of a newborn baby—was not something tangible, like the length of a leg, the color of a fingernail, or the noise of a heart murmur. A baby's condition is an intangible clinical concept or "construct," somewhat like anxiety or congestive heart failure, that is not directly observed as an individual entity, but that can be perceived, interpreted, and rated from a set of pertinent observed phenomena.
Feinstein AR. Multi-item "Instruments" vs Virginia Apgar's Principles of Clinimetrics. Arch Intern Med. 1999;159(2):125–128. doi:10.1001/archinte.159.2.125