Data Quality

Data Quality

Make bad data loud: caught at the boundary where it enters, blocking what must not ship, alerting someone who owns the fix — instead of being discovered by a stakeholder in a dashboard. Boundaries: generating fake/test data is test-data-strategy; designing the schema is data-modeling; this skill keeps the real data flowing through pipelines trustworthy.

Workflow

Step 1: Start From Incidents, Not Inventory

Don't blanket every column with tests. List the recent data incidents (wrong dashboard numbers, silent breakage) and the tables where an error costs real money or trust — revenue marts, ML features, anything an exec reads. Each known symptom maps to a check: nulls → not_null, duplicates → unique, staleness → freshness, broken joins → relationships. Wins from day one, and the suite grows the same way tests do after bugs — every new incident adds a check that would have caught it.

Step 2: Check at the Right Layer — Boundary First

Where a check runs matters as much as what it checks:

• Ingestion boundary (sources): schema, freshness, volume, primary-key integrity — catch bad inputs before transforms consume them. One failed source check beats fifty downstream test failures at 6 a.m.
• Transform layers (staging → marts): uniqueness/not-null on the grain of each model, relationship tests on joins, accepted values on enums.
• Consumer edge (marts/metrics): business-rule assertions — "revenue is never negative", "row count within N% of the 7-day average".

In dbt this is: source freshness + source tests, generic tests on models (unique, not_null, relationships, accepted_values), singular SQL tests for business rules. Dedicated tools (Great Expectations, Soda) earn their keep outside dbt-land or when non-engineers must own the rules: references/tooling.md.

Step 3: Assign Severity — What Blocks vs What Warns

A suite where everything fails the build gets muted within a month; a suite where nothing blocks is decoration. Decide per check:

• error — data is unusable or contaminating (duplicate keys in a revenue mart, failed contract at ingestion): the pipeline stops, downstream models don't build on poisoned input.
• warn — degraded but usable (slightly stale, volume dip within reason): alert, keep flowing.

dbt: severity: config with error_if/warn_if thresholds. Revisit severities after each incident and each false alarm.

Step 4: Route Failures to an Owner

A failing check nobody sees is identical to no check. Every table (or domain) has a named owner; alerts go where that owner works (Slack channel per domain, pager only for error-severity on critical marts), and each alert says what failed, on which table, and the blast radius (Step 5). Track time-to-detection and time-to-fix for data incidents — that's the metric this whole skill improves (observability-design for the SLO framing).

Step 5: Use Lineage to Scope Blast Radius

When a check fails, the first question is "what downstream is now wrong?" — lineage (dbt's DAG, dbt ls --select my_model+, or a catalog tool) answers which models, dashboards, and consumers sit downstream of the failure. Use it to: prioritize fixes by consumer impact, notify affected dashboard owners proactively, and decide what to rebuild after the fix. If you can't answer "what breaks if this table is wrong?", invest in lineage before more checks.

Step 6: Contract the Upstream Boundary

Downstream tests detect what upstream producers break; contracts prevent it. When an external team owns the source: agree schema + semantics + SLA explicitly (a versioned artifact — dbt model contracts, JSON Schema/protobuf in a registry), enforce at ingestion so violations fail loudly at entry, and shift left — contract validation in the producer's CI, so a breaking rename fails their build, not your 6 a.m. run. Breaking changes get a version bump and a deprecation window, coordinated like an API change (dependency-impact-analysis thinking, applied to data). Back it with schema-drift detection and volume/distribution anomaly checks for what enforcement can't see.

Principles Applied

• YAGNI: checks earn their place by a failure mode you've seen or can't afford — blanket-testing every column produces alert fatigue, the failure mode that kills quality programs.
• Fail loudly at the boundary: the cost of bad data grows with every layer it flows through undetected.
• DRY: one contract at the source replaces N teams' defensive checks against the same breakage.

Cross-Skill References

• data-pipeline-design — the pipeline these checks are embedded in
• data-modeling — constraints and grain the checks assert (unique keys exist by design)
• test-data-strategy — generating synthetic/fixture data for testing pipelines
• observability-design — alerting design, SLOs, avoiding alert fatigue
• incident-response — when bad data has already reached production consumers