2. Injury scorecard fundamentals
An NCAP injury scorecard is not a single number — it is a structured ledger of physical measurements, each mapped to a clinical severity score, then aggregated into a rating. Before you can compute a vehicle rating, you must understand three layers: where the measurement is taken (body regions), what is measured (injury metrics), and how that measurement is converted into a comparable severity value (the AIS scale). Getting any one of these layers wrong propagates directly into the star rating.
Body regions: the scorecard's coordinate system
Global NCAP scorecards partition the occupant into discrete body regions so that injuries in different anatomical areas can be tracked and weighted separately. The standard partition used across frontal and side impact protocols is:
- Head — including the skull and brain; assessed via head acceleration and HIC.
- Neck — axial force, shear force, and bending moment.
- Chest — rib deflection, viscous criterion (VC), and acceleration.
- Abdomen — force and deflection, particularly in side impact.
- Pelvis — force and pubic symphysis loading in side impact.
- Upper and lower extremities — femur force, knee loading, tibia index.
Each region has its own set of transducers and its own set of injury reference values. A chest deflection of 40 mm and a femur force of 4 kN are not comparable quantities — they only become comparable once each is converted to an AIS severity level. This is why the scorecard is organized region-by-region rather than as a flat list of channels.
Injury metrics: what the transducers actually report
The raw channels recorded during a crash test are physical quantities. The most important ones in NCAP frontal and side impact are:
| Metric | Region | Physical meaning | Typical NCAP reference |
|---|---|---|---|
| HIC15 | Head | Resultant head acceleration integrated over a 15 ms window | ~700 (AIS 3 threshold region) |
| a3ms | Head | Peak resultant acceleration sustained for 3 ms | ~80 g |
| Chest deflection | Chest | Rib cage compression (mm) | ~50–60 mm (varies by protocol) |
| VC | Chest | Viscous criterion: deflection rate × compression | ~1.0 m/s |
| Neck Fz / My | Neck | Axial force and bending moment | Protocol-specific |
| Femur force | Lower extremity | Axial compressive load (kN) | ~4–10 kN |
Two conventions matter here. First, HIC is computed from the resultant acceleration at the head centre of gravity, not from a single axis — the formula is HIC = max over t₁,t₂ of [(t₂−t₁) · (1/(t₂−t₁)) ∫ a dt]^2.5, with the window capped at 15 ms for HIC15. Second, chest deflection is measured relative to the spine box, so the sign convention (compression positive) must be stated explicitly and held consistent across all channels in the scorecard.
The AIS scale: from physical quantity to clinical severity
The Abbreviated Injury Scale is an ordinal clinical scale from 1 to 6, where 1 is minor and 6 is currently untreatable/fatal. In NCAP context, each metric is mapped to an AIS level through a piecewise-linear risk curve derived from cadaver and PMHS testing. The mapping is monotonic: higher HIC, higher deflection, higher force → higher AIS.
The critical concept for scorecard work is that AIS is a severity label, not a probability. A chest deflection that maps to AIS 3 means "this loading is consistent with a serious injury of that region," not "there is a 30% chance of injury." NCAP protocols then apply a second transformation — an injury risk curve — to convert AIS level into a probability, and it is those probabilities that are summed into the overall score. Confusing the two is one of the most common errors in scorecard interpretation.
How the three layers combine
The diagram below shows the data flow from transducer to scorecard entry. Each body region has its own metric set, each metric is mapped through its own AIS curve, and the resulting AIS levels are what the rating calculation consumes.
Reading a scorecard entry in practice
Consider a worked example from a frontal offset test. The head channel reports a peak resultant acceleration of 72 g sustained over 3 ms, and the computed HIC15 is 540. The chest IR-TRACC reports a peak deflection of 44 mm with a VC of 0.82 m/s. The neck load cell reports a tension force of 2.9 kN and a bending moment of 42 Nm.
Applying the protocol's AIS curves: HIC 540 falls in the AIS 1 band (typically HIC 135–519 is AIS 1, 520–899 is AIS 2, so 540 sits just inside AIS 2). Chest deflection of 44 mm maps to AIS 2, and VC 0.82 is below the AIS 3 threshold of 1.0, so the chest entry is AIS 2. Neck Nij of approximately 0.6 maps to AIS 1. The scorecard therefore records Head AIS 2, Chest AIS 2, Neck AIS 1, Femur AIS 1 — and it is these four labels, not the raw numbers, that feed the rating calculation.
What to verify before trusting a scorecard
Three checks catch most scorecard errors. First, confirm the channel-to-region mapping: a chest deflection channel accidentally routed to the abdomen row will silently produce a plausible but wrong AIS. Second, confirm the curve version: HIC thresholds and chest deflection bands differ between protocol years, and the scorecard must state which revision was applied. Third, confirm the aggregation rule: some protocols take the maximum AIS per region, others sum risk probabilities across regions — these produce different ratings from identical raw data. A scorecard that does not state its curve version and aggregation rule is not reproducible and should not be used as a rating input.
With the three layers — body regions, injury metrics, and AIS mapping — clearly separated, the next step is to see how the aggregated AIS levels are converted into the star rating itself.