docs/superpowers/specs/2026-08-28-destination-measures-design.md

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+# Destination Measures — Design
+
+**Date:** 2026-08-28
+**Status:** awaiting review
+**Scope:** secondary school detail pages only
+
+## Goal
+
+Say what happened to a school's leavers after they left. Two sections on the
+secondary template:
+
+- **After Year 11** — every secondary, from the KS4 destination measures
+- **After the sixth form** — sixth-form schools only, from the 16-18 measures
+
+This replaces the "Post-16 destination data coming soon" placeholder standing in
+`nextjs-app/components/school/SecondaryAdmissionsSection.tsx:117` since the exam
+phase taxonomy work, and fills the `ks5_destinations_pct` slot specified but
+never built in `2026-07-07-exam-phase-taxonomy-design.md:201`.
+
+Mockup, with all three data states live:
+<https://claude.ai/code/artifact/5be149d6-252f-473c-9a4f-4c36b05161b0>
+
+## The finding that shapes everything
+
+**Suppression is per cell, and the cells sum to the cohort.**
+
+DfE withholds a figure it considers disclosive by writing `c`. It does this at
+the level of an individual destination category, not the whole school, and it
+publishes the cohort total alongside. The categories form a clean partition. So
+where exactly one category is suppressed, subtracting the published ones from the
+cohort recovers it exactly.
+
+Verified against three real schools in the 2022/23 file:
+
+| School | URN | Withheld | Recovers to |
+|---|---|---|---|
+| North East Futures UTC | 145900 | School sixth form | **3 pupils** |
+| Whitley Bay High School | 108638 | Further education | **18 pupils** |
+| St Matthew's RC High School | 148389 | School sixth form | **4 pupils** |
+
+Those are the precise numbers the `c` exists to hide, and in a random 400-school
+sample **22% of mainstream secondaries** have exactly one suppressed category in
+their disadvantaged group. This is the normal case, not an edge case.
+
+Two rules follow, and everything else in this document is downstream of them.
+
+**R1 — Never render a derived remainder.** Not as a number, and not as a bar
+segment: a segment sized by the residual can be read straight off the axis. Where
+any category in a pupil group is suppressed, the page shows the published
+categories, says the rest are withheld, and stops.
+
+**R2 — Never aggregate across a suppression boundary.** A group total is
+publishable only when DfE published that total itself, or when the aggregate
+spans **two or more** suppressed cells. Summing published components to fill a
+gap is R1 with extra steps.
+
+**R3 — The three pupil groups are one disclosure surface, not three.**
+Disadvantaged and Not-known-to-be-disadvantaged partition All pupils, so
+rendering any *two* of them recovers the third. Where a category is suppressed in
+the disadvantaged group, it must therefore also be withheld from **all other
+pupils** — the all-pupils view is the primary one and keeps it.
+
+This costs almost nothing, because DfE already applies the same masking: across
+the sample, 493 of 498 suppressed disadvantaged cells were suppressed in the
+other group too. The mart enforces the remaining 5, which fell on 2 schools of
+262. **The all-pupils bar is unaffected** — masking the whole page wherever the
+disadvantaged group is thin would remove the bar from 80% of schools, and is not
+what this rule says.
+
+R1 and R2 both hold within a group and still leak across the switch, which is why
+R3 is stated separately.
+
+### The convention that would break this quietly
+
+`macros/safe_numeric.sql` coerces every EES sentinel — `z`, `c`, `x`, `q`, `u` —
+to `NULL`, deliberately and correctly for attainment, where "suppressed" and "no
+data" are equally unrenderable. Here they are not the same thing: one must print
+*withheld*, the other must print nothing at all, and the difference is what keeps
+R1 enforceable.
+
+**`safe_numeric` must not be used on destination counts.** The staging model
+keeps the sentinel in a companion status column. This is the single most likely
+way for this feature to regress into a disclosure, so it gets its own dbt test.
+
+## What is actually available
+
+Measured against the EES public API (open, no key). Both datasets carry
+`geographicLevel: School` with `urn` on every location option, so the join to
+`dim_school` is direct.
+
+| | KS4 | 16-18 |
+|---|---|---|
+| Dataset id | `019d4f41-22d1-71b2-a1a7-f3b91026815b` | `019d4e73-6440-7523-b60c-bfab1ad4a30d` |
+| Rows | 1,871,739 | 3,862,658 |
+| Institutions | 4,946 | 3,065 |
+| Time periods | 2009/10–2022/23 | 2016/17–2022/23 |
+
+**Destination categories (KS4).** School sixth form · Sixth form college ·
+Further education · Other education destination · Sustained apprenticeships (with
+level breakdown) · Sustained employment destination · Not recorded as a sustained
+destination · Activity not captured. Plus the aggregates `Sustained education
+destination` and `Sustained education, employment & apprenticeships`.
+
+**16-18 adds** UK higher education institution and FE split by level, which is
+what makes the post-16 section worth having.
+
+**Breakdowns.** `Disadvantage Status` gives Disadvantaged / Not known to be
+disadvantaged / Total — exactly the three-way switch. Sex, ethnicity, FSM status,
+prior attainment and SEN provision also travel in the same table; we ingest none
+of them.
+
+**Indicators.** Both counts and percentages, plus the cohort size. Bar widths use
+the counts — the published percentages do not sum to 100.
+
+### Coverage, and what degrades
+
+Random 400-school sample, 2022/23, mainstream secondaries (n=262):
+
+| View | Published | Consequence |
+|---|---|---|
+| All pupils, all categories | **100%** | Full bar works everywhere |
+| Disadvantaged, headline rate | 95% | Gap panel works |
+| Disadvantaged, three grouped cards | 68% | Degrades card by card |
+| Disadvantaged, all six categories | **20%** | Bar unusable for this group |
+
+Special schools and alternative provision are far worse: 13% and 41% respectively
+have the whole cohort suppressed even for all pupils. The empty state is
+load-bearing, not defensive.
+
+## The display
+
+Question-led. Three cards over one bar, with the cards acting as a lens on the
+bar rather than a summary beside it — hovering a card dims the bar, table and
+England reference to the categories that card is built from. The full mockup is
+linked above; what matters for implementation:
+
+**The headline is not the sustained rate.** That figure sits between 92% and 97%
+for nearly every school in England. The mix is what varies, so the mix leads.
+
+**The grouping is ours, not DfE's.** "Academic route" = school sixth form +
+sixth-form college; "College" = FE and other colleges; "Work" = apprenticeship +
+employment. This is the most arguable thing on the page, so it lives in one place
+in `lib/destinations.ts`, is explained in a tooltip, and is reversible in one
+edit.
+
+**The absence is hatched neutral, never a colour.** "Activity not captured" means
+no record in the sources DfE holds — it includes independent schools, moving
+abroad and private training. Colouring it as a bad outcome would be a factual
+error rendered in CSS. The hatch also fixes a real contrast problem: neutral
+against the employment blue failed CVD separation at ΔE 7.6, and texture is the
+secondary encoding that rescues it. Every other adjacent pair clears ΔE 10.9
+under protanopia.
+
+**Colour tokens.** Education is one hue in three steps (school-like to
+college-like); apprenticeship and employment are separate hues. Six new tokens in
+`globals.css`, defined in both themes, per the existing token discipline.
+
+**The disadvantage split rides the same control.** One visualisation serving
+three cohorts, with the England reference repointing to the matching national
+group. The gap statement stays visible below the bar whatever is selected,
+because a gap nobody clicks on is a gap nobody sees.
+
+## Data model
+
+### Extraction
+
+A new `tap-uk-ees-destinations` extractor, separate from `tap-uk-ees`. The
+existing tap downloads a release ZIP and reads a CSV inside it; the destinations
+files are far larger than we need and the query API filters server-side, so this
+one POSTs to `/v1/data-sets/{id}/query` and pages through results.
+
+With every dimension pinned — destination measures, disadvantage status, sex
+Total, characteristic topic Total — one year returns **252,610 rows** across all
+geographic levels. Three school-level years is comfortably tractable.
+
+Pinning is mandatory, not an optimisation: leaving the characteristic dimensions
+unconstrained returned 45 rows where 9 were wanted, because every breakdown
+shares one table.
+
+The tap emits the raw value as text. **It does not coerce `c`.**
+
+### Staging
+
+`stg_ees_ks4_destinations` / `stg_ees_ks5_destinations`. Each raw value becomes
+two columns:
+
+```sql
+case when raw ~ '^-?[0-9]+(\.[0-9]+)?$' then raw::numeric end as pupils,
+case
+    when raw ~ '^-?[0-9]+(\.[0-9]+)?$' then 'published'
+    when lower(trim(raw)) = 'c'        then 'suppressed'
+    else 'not_applicable'
+end as status
+```
+
+### Marts
+
+`fact_ks4_destinations` and `fact_ks5_destinations`, **long format**:
+
+```
+urn, year, pupil_group, destination_category, cohort_pupils, pupils, percentage, status
+```
+
+This departs from the wide house pattern (`fact_ks4_performance` and friends) on
+purpose. `pupil_group` is a genuine third dimension; going wide would need three
+sets of every column, and R2 is far easier to test on rows than on columns.
+
+Roughly 8 categories × 3 groups × 4,946 schools × 3 years ≈ 356k rows.
+
+`fact_destination_national` carries the same grain for England, so the page's
+England reference repoints with the switch.
+
+### dbt tests
+
+- `assert_destinations_no_derived_remainder` — for every (urn, year,
+  pupil_group) with exactly one suppressed category, assert no aggregate row
+  exists that would let the residual be recovered. **This is the R1 guard.**
+- `assert_destinations_group_masking` — for every (urn, year, category), if the
+  disadvantaged group carries `suppressed`, so does the other-pupils group.
+  **This is the R3 guard**, applied in the mart so no consumer can reach an
+  unmasked combination.
+- `assert_destination_status_null_agreement` — `pupils is null` wherever
+  `status != 'published'`, and never null where it is.
+- `assert_destinations_join_dim_school` — no orphaned URNs, matching the
+  existing `assert_no_orphaned_facts` pattern.
+
+## API
+
+`GET /api/schools/{urn}` gains a `destinations` block:
+
+```json
+{
+  "ks4": {
+    "cohort_year": "2022/23",
+    "published": "2026-04",
+    "groups": {
+      "all":           { "cohort": 180, "categories": [ … ], "aggregates": { … } },
+      "disadvantaged": { … },
+      "other":         { … }
+    }
+  },
+  "ks5": { … }
+}
+```
+
+Each category carries `pupils`, `percentage` and `status`. **The serialiser never
+emits a computed remainder**, and a backend test asserts that a group containing a
+suppressed category serialises no total that closes the gap.
+
+`null` for the whole block where nothing is published — the frontend renders the
+empty state from its absence, not from a sentinel.
+
+## Frontend
+
+| File | Kind | Job |
+|---|---|---|
+| `lib/destinations.ts` | pure | Category list, the academic/college/work grouping, `canAggregate()` enforcing R2, percentage derivation from counts |
+| `components/school/DestinationsSection.tsx` | server | Section shell, renders **all pupils** into the HTML |
+| `components/school/DestinationsView.tsx` | client | Cohort switch, card↔bar linkage |
+| `components/school/Post16DestinationsSection.tsx` | server | Year 13 section, sixth-form schools only |
+| `app/globals.css` | tokens | Six destination colours, both themes |
+
+Server-first matches the directory's existing discipline — every component in
+`components/school/` is a server component except `AdmissionsViewToggle`, which
+is the precedent this follows. All-pupils figures are in the HTML for crawlers
+and for no-JS; only the switch and the hover linkage need the client.
+
+`lib/schoolSections.ts` gains `hasKs4Destinations` / `hasKs5Destinations` flags
+and the nav items, following the existing `computeSchoolFlags` pattern.
+
+**Placement** on the secondary template: GCSE results → After Year 11 → After the
+sixth form → admissions. Destinations follow attainment because they answer "and
+then what happened".
+
+**Dating.** The latest destination year is 2022/23, published April 2026, while
+the site's newest KS4 year is 2024/25. The section header states its own cohort
+year, or it reads as stale data next to the GCSE section above it.
+
+## Edge states
+
+| State | Frequency | Behaviour |
+|---|---|---|
+| Whole cohort suppressed | 13% of special, 41% of AP | Section renders the explanation, no chart |
+| Some categories withheld | 80% of disadvantaged views | Cards degrade individually; **no bar**; table marks withheld rows |
+| Disadvantaged group suppressed entirely | 5% | Switch drops to two options, gap panel not rendered |
+| No sixth form | — | Post-16 section not rendered at all — absence is correct, a "no data" placeholder would imply something is missing |
+| School too new | — | "First figures expected in 2026", not a bare no |
+
+## Testing
+
+Per CLAUDE.md, user-facing behaviour extends `e2e/` in the same PR.
+
+**Unit** — `lib/destinations.ts` is where R1 and R2 live, so it carries the
+heaviest tests: `canAggregate()` refuses a group containing one suppressed cell,
+allows one spanning two, and the bar builder refuses to emit segments for any
+group with suppression. These are the tests that must fail loudly if someone
+later "fixes" a gap in the chart.
+
+**dbt** — the three tests above.
+
+**Backend** — the serialiser emits no closing total for a partially suppressed
+group.
+
+**E2E** — a school with full data renders three cards and a bar; a school with a
+partially suppressed disadvantaged group renders the withheld state and **no bar
+element**; a suppressed school renders the explanation; a school with no sixth
+form renders no post-16 section.
+
+Note the staging caveat: mart changes are inert until the Airflow pipeline runs,
+and the staging E2E gate runs post-merge.
+
+## Out of scope
+
+- **Compare view and rankings.** The long mart shape supports both; neither is
+  built here. Flagged because "% to a school sixth form" is a plausible rankings
+  metric and the mart shape should not have to change to allow it.
+- **Ethnicity, sex, SEN and prior-attainment breakdowns.** Available in the same
+  file, ingested deliberately not at all — each is a separate editorial decision
+  about what a school page should assert.
+- **Longer term destinations** (3 and 5 years out) and **Progression to higher
+  education** — separate publications, worth a later look for sixth forms.
+- **Primary schools.** No KS2 destination measures publication exists; DfE
+  tracking starts at KS4. Naming the secondaries a primary's leavers go to needs
+  the National Pupil Database, which is not publishable at that grain.
+
+## Risks
+
+**A later change reintroduces the disclosure.** The likeliest route is someone
+applying `safe_numeric` to a destination column for consistency, or adding a
+`coalesce` in a mart. Mitigation is the dbt test plus the unit tests on
+`canAggregate()` — the rule has to be executable, not documentary.
+
+**The two-year lag reads as staleness.** Mitigated by dating the cohort in the
+section header rather than only in a tooltip.
+
+**Sixth-form retention will be misread.** "41% went to a school sixth form" says
+nothing about *which* school. The published file reports destination type, never
+destination institution. Copy must never imply "stayed on here", and the tooltip
+should say so.
+
+**Section length.** The secondary template is already long and this adds two
+sections. If it becomes a problem the post-16 section is the one to collapse
+behind a disclosure, not the Year 11 one.
+
+## Open questions
+
+1. Is the disadvantage split its own section or a sub-block inside the
+   destinations section? Modelled as a sub-block; it is the most differentiating
+   figure on the page and the most easily misread on a small cohort.
+2. Do we ingest the apprenticeship level breakdown (intermediate / advanced /
+   higher) now, or collapse to one apprenticeship figure and revisit? Collapsed
+   in this design.