MOT Risk Index Evidence Capsule: Great Britain vehicle-family and defect-group aggregates, 2024

v3 (2026-09-24) corrects v1 and v2. The DVSA 2024 source files repeat many test records (66,857,355 rows hold 42,598,624 distinct tests); v1 and v2 counted the repeats. v3 keeps one record per test ID: class-4 initial tests 33,012,215 (previously 52,245,925), national initial-failure rate 28.28% (previously 26.01%). Coverage grows from 34 to 625 published vehicle families. Method and changelog: methodology, changelog. Aggregated 2024 Great Britain MOT initial-test outcomes for 34 vehicle families, derived from the DVSA Anonymised MOT tests and results bulk dataset (Open Government Licence v3.0). The headline metric is the age-adjusted MOT initial-failure ratio: observed first-time failures (F + PRS) divided by the count expected when frozen national class-4 rates are applied to the family's own age strata, multiplied by 100 (100 = expected for that age profile). A second table gives family-by-defect-group failure-item rates per 100 tests, excess in percentage points and over-representation ratios against the national benchmark, for eleven prespecified top-level defect groups. Disclosure control: cells with n < 1,000 are omitted; rows with n between 1,000 and 5,000 are flagged low_sample; where exactly one cell in a family slice is suppressed, the next-smallest is also removed so the first cannot be recovered by subtraction. No vehicle-level records, registration marks or geographic slices are included. These are observational inspection outcomes, not causal estimates; mileage is not equalised between families. Canonical dataset page: motriskindex.co.uk.