How VPA peer norms are computed

VPL scores are ordinal (1–5), not continuous. Reporting means, standard deviations, or z-scores on ordinal data creates a false sense of precision that would inflate small differences and understate uncertainty. We use methods appropriate for ordinal ranking data:

• Empirical CDF with mid-rank ties. A student's percentile is the proportion of peers scoring below their VPL, plus half the proportion scoring exactly their VPL. This is the standard, unbiased treatment for tied ordinal data.
• Wilson score 95% confidence intervals on the underlying proportion. When a sample is small, we show the interval rather than a false-precision point estimate.
• No cross-protocol pooling. Norms are stratified by protocol_version. When the assessment changes, old norms are retired — not silently mixed with new data.

Every submission in the norming pool must meet all of the following:

• Complete per-skill VPL scoring (partial assessments are excluded).
• Age band is recorded (so peer matching is possible).
• Submitted by a non-admin account (test data from developer/admin accounts is excluded).
• At most one submission per student per protocol version — the most recent is kept, so a student reassessed six weeks later doesn't double-count.

Anonymous submissions (no linked student record) each count once. Grade and setting are used for finer peer-matching only when the finer cell independently clears the sample-size gate.

No percentile is displayed until a peer cell reaches n = 30. Below that, the results screen shows a "Building norms" progress line — the running count out of 30. Between 30 and 100, results are labeled "small sample" and the 95% CI is displayed alongside the point estimate.

This is deliberate. Reporting a "62nd percentile" based on 8 comparisons is worse than reporting nothing at all — it launders noise as authority.

Norms recompute nightly from the raw vpa_submissions table via a deterministic SQL aggregation function (refresh_vpa_norms). The function is idempotent — running it twice produces the same table. The refresh timestamp appears on every peer-comparison panel.

The VPA is a criterion-referenced clinical framework authored by an occupational therapist for school-based and clinic OT practice. It is in a v1.0 Beta norming phase: percentiles publish per age-band cell only after n ≥ 30, and each cell carries a Wilson 95% CI until it reaches n = 100. The VPA has not undergone an external psychometric validation study, and is not marketed as a standardized diagnostic test. It is intended to structure OT clinical reasoning about functional visual performance, not to replace comprehensive vision examination by a licensed eye-care provider when refractive or ocular pathology is suspected. A live reliability panel (above) reports Cronbach's α and test-retest r per item-bank version as retest pairs accumulate.

The VPA framework, item design, and intervention packets draw on the following bodies of literature. Citations are provided so clinicians and researchers can trace the reasoning behind each design choice.

Citations are provided as the scholarly grounding for the framework and item design. They are not endorsements of the VPA by any of the cited authors or organizations.