# Utah Vehicle Health project charter

## Prototype status

**Presentation-ready development prototype.** Every analytical finding and every
dashboard value comes from the private, page-sampled 10,000-vehicle development
cohort. These values are **sample results, not population estimates**. They must
not be used to estimate statewide or county prevalence, create county rankings,
or make decisions about an individual vehicle.

## Product promise

Utah Vehicle Health explains how information available before an inspection
relates to the first-attempt non-pass outcome of a returning vehicle's next
inspection episode.

The product measures an emissions-inspection outcome. It does not measure
overall vehicle health, mechanical reliability, safety, roadworthiness, or
legal compliance.

## Primary research question

> Using only information available before an inspection episode begins, how
> well can a calibrated logistic regression estimate whether a returning
> vehicle's next episode will have a first-attempt non-pass outcome?

An episode groups consecutive attempts for the same private vehicle token when
the gap is 30 days or less. The target is the first attempt of a new episode,
not a rapid retest. A vehicle is eligible only after at least one completed
prior episode.

## Binary target

- `0 - pass`: normalized `PASS` or `P`
- `1 - non-pass`: normalized `FAIL`, `F`, `REJECT`, or `ABORT`
- unlabeled: blank, null, and unrecognized results

Reject and abort can reflect process or readiness conditions rather than a
mechanical failure. The prototype therefore says **non-pass**, never “vehicle
failure,” when referring to the combined target. Multiclass outcome modeling
and fail-only sensitivity analysis are outside the finished prototype scope.

## Data and product boundary

The development cohort contains complete inspection histories for up to 10,000
sampled vehicles from participating Utah county/source feeds. Page sampling
over-represents vehicles with more inspection records, so neither the cohort nor
its dashboard aggregates are population-representative. The feeds do not cover
all 29 Utah counties.

The final model is inspection-history only. DMV enrichment, rich current-test
OBD fields, station effects, cold-start prediction, and individualized lookup
are outside scope. The static dashboard publishes only rounded, suppressed
sample aggregates and pre-2025 development diagnostics.

## Final model and benchmark

The final model is regularized logistic regression (`C=0.03`) with Platt
probability calibration fit on the 2024 development-sample partition. A
histogram gradient-boosted tree is retained only as a nonlinear benchmark. It
is not a second final model and is not used to drive the product.

## Completed deliverables

1. Read-only extraction with private vehicle pseudonymization.
2. Leakage-safe episode and point-in-time feature mart.
3. Training-prevalence and previous-episode baselines.
4. Calibrated logistic regression as the final model.
5. Histogram gradient boosting as a benchmark only.
6. Chronological development evaluation with an explicit one-time 2025 gate.
7. Suppression-reviewed static dashboard assets with fail-closed validation.
8. Model card, final report, presentation materials, and dashboard-only Bolt
   deployment instructions.

## Acceptance criteria

- The prediction unit remains a returning vehicle's next-episode first attempt.
- Model features exist before that episode begins.
- The calibrated logistic model beats both simple baselines on development-
  sample PR-AUC and Brier score.
- The boosted tree is presented only as a benchmark.
- Every displayed value is labeled as a private-sample result and not a
  population estimate.
- County views communicate feed coverage and sample context; they do not claim
  population rankings or causal county differences.
- No VIN, plate, ZIP, station, technician identifier, raw JSON, credential,
  operational record, private token, or row-level prediction reaches the
  dashboard or presentation materials.
- The dashboard contains no vehicle-level estimator, input form, or prediction
  service.

## Non-goals

- Predicting the result of a first-observed vehicle
- Diagnosing, certifying, or guaranteeing an individual vehicle outcome
- Ranking counties, programs, stations, technicians, owners, or vehicles
- Making causal claims about geography, vehicle makes, or inspection programs
- Accepting VIN, plate, address, station, or diagnostic inputs
- Publishing population prevalence from the development sample
- Shipping a production decision service or live database connection
- Completing DMV enrichment, four-class modeling, or failure-to-pass journeys

## Evaluation status

The explicit 2025 gate was opened once after model specifications were frozen
from pre-2025 data. No 2025-informed model change was made. The 2025 values are
therefore a one-time **development-sample holdout**, not a pristine future test
and not population performance. See [development_results.md](development_results.md)
for the audit trail and [modeling_protocol.md](modeling_protocol.md) for the
frozen evaluation contract.