When the ruler keeps changing
Much of how we assess movement still depends on the trained human eye. A clinician watches a patient rise from a chair, walk a few meters, or hold a balance pose, then translates that observation into a score on an ordinal scale. It works, but it carries a quiet problem: the ruler keeps changing. Two clinicians can observe the same patient and arrive at different scores, and the same clinician can score differently on a busy afternoon than on a quiet morning. This isn't a new finding. When three expert observers rated videotaped gait, Krebs and colleagues (1985) found identical ratings in only about two-thirds of observations and judged observational gait analysis "convenient, but only moderately reliable." A later study in which 54 licensed physical therapists rated the same videotaped patients found agreement that was only slight to moderate, and called for greater standardization (Eastlack et al., 1991).
Why ordinal scales blur progress
Many standard assessments compress rich, continuous movement into a handful of categories — a 0 to 4 rating, a "mild/moderate/severe" label. These scales are practical and widely validated, but they are coarse by design. Tremor offers a well-measured example: one point on a typical 0–4 tremor rating scale corresponds to a substantial, roughly multiplicative change in measured tremor amplitude (Elble et al., 2006). A patient can improve meaningfully without moving up a whole category, and that progress becomes invisible in the chart. When the measurement can't resolve small changes, it's hard to know whether a plan of care is working.
The cost of the gap between visits
Subjective assessment also depends on memory. Between visits, both patient and clinician rely on recall to judge whether things are better or worse. Recall is fallible, and it tends to be shaped by how the patient feels on the day they happen to be in the room — not by the underlying trend.
Toward a steadier measurement
The goal isn't to remove clinical judgment; it's to give it a steadier foundation. Objective, quantitative movement measurement provides the same yardstick every time, for every clinician, and resolves the small changes that ordinal scales miss. Used alongside experienced clinical reasoning, consistent measurement turns "I think they're improving" into something a care team can see, compare, and act on.
Reliability also determines how small a change you can trust — see is it real change? Reading progress against meaningful-change thresholds.
References
- Krebs DE, Edelstein JE, Fishman S. Reliability of observational kinematic gait analysis. Physical Therapy. 1985;65(7):1027-1033. doi:10.1093/ptj/65.7.1027
- Eastlack ME, Arvidson J, Snyder-Mackler L, Danoff JV, McGarvey CL. Interrater reliability of videotaped observational gait-analysis assessments. Physical Therapy. 1991;71(6):465-472. doi:10.1093/ptj/71.6.465
- Elble RJ, Pullman SL, Matsumoto JY, et al. Tremor amplitude is logarithmically related to 4- and 5-point tremor rating scales. Brain. 2006;129(10):2660-2666. doi:10.1093/brain/awl190