Ab Testing

Scope Boundaries

• Use when product, pricing, ranking, recommendation, or policy changes may impact user/business KPIs and causal validation is required.
• Use proactively when metric impact is uncertain, stakeholder opinions conflict, or ship/no-ship criteria are not explicit.
• Use when canary metrics alone are insufficient to support a decision.
• Do not use for deterministic functional verification; use testing-*.
• Do not use for long-term reliability telemetry design; use observability-*.

Goal

Produce causally valid, operationally safe, and decision-ready experiment outcomes.

Shared Experiment Contract (Canonical)

• Use references/ab-testing-governance-contract.md as the primary reference for recommended structure.
• Optional consistency checks (only if your repository enforces manifest validation):
  • python3 scripts/validate_ab_testing_contract.py --manifest <path/to/manifest.json>
• Start from valid templates in assets/:
  • assets/ab-pln-manifest.valid.json
  • assets/ab-dec-manifest.valid.json
• Use decision-rule details in:
  • references/decision-threshold-playbook.md
• Do not define local ID formats, lifecycle states, or gate rules in this file.

Implementation Templates

• Experiment charter template:
  • assets/ab-experiment-charter-template.md
• Decision record template:
  • assets/ab-decision-record-template.md

Inputs

• Proposed change and explicit decision to be made (ship, iterate, rollback, hold).
• Business objective and target KPI with current baseline/variance estimates.
• Traffic budget, experiment window constraints, and seasonality considerations.
• Risk posture for false positives/false negatives and required confidence level.
• Guardrail metrics (reliability, latency, abuse, revenue-risk, support load).
• Data contract and instrumentation readiness for all required events.

Outputs

• Experiment charter: hypothesis, population, assignment unit, randomization method, and contamination controls.
• Analysis plan: primary metric, secondary metrics, guardrails, MIN_DETECTABLE_EFFECT, confidence/precision targets, and decision gates.
• Runbook for monitoring, stop/escalation criteria, and incident response during experiment.
• Decision record with effect sizes, uncertainty bounds, segmentation caveats, and rollout recommendation.

Decision Framework

• Define explicit costs for wrong decisions:
  • FALSE_POSITIVE_COST: shipping harmful change.
  • FALSE_NEGATIVE_COST: rejecting beneficial change.
• Set statistical strictness from decision risk, not preference:
  • Higher FALSE_POSITIVE_COST -> stricter false-positive control.
  • Higher FALSE_NEGATIVE_COST -> higher power and/or larger sample.
• Derive MIN_DETECTABLE_EFFECT from minimum business-material impact worth shipping.
• Use one primary decision metric; other metrics are supportive or guardrails.
• Pre-register ship criteria before exposure starts; do not redefine after peeking.

Workflow

1. Frame the decision and hypothesis.
   • Specify target behavior change, affected population, and unacceptable harm scenarios.
   • Record decision owner and approvers for release risk.
2. Define metrics and decision rules before launch.
   • Choose exactly one primary metric for go/no-go.
   • Define guardrails and hard stop conditions (for example reliability or revenue damage).
   • Document attribution window, aggregation level, and missing-data handling.
3. Design assignment and contamination controls.
   • Select assignment unit (user, session, org, device) based on interference risk.
   • Define randomization strategy and stratification requirements.
   • Prevent spillover between treatment/control where feasible.
4. Plan evidence requirements.
   • Estimate sample size from baseline, variance, MIN_DETECTABLE_EFFECT, and confidence/power targets.
   • Define fixed-horizon or sequential analysis plan before runtime.
   • Ensure minimum runtime captures weekday/weekend and campaign effects.
5. Validate instrumentation and randomization integrity.
   • Run event schema and logging checks before exposure ramp.
   • Monitor sample ratio mismatch (SRM) and stop if assignment integrity is broken.
6. Execute with operational safeguards.
   • Ramp traffic gradually with explicit hold points.
   • Continuously evaluate guardrails and trigger rollback on hard breaches.
7. Analyze according to the pre-registered plan.
   • Report effect size and uncertainty interval for primary metric.
   • Treat unplanned segment findings as exploratory unless pre-registered.
   • Apply multiplicity control when multiple hypotheses can drive decisions.
8. Make and document the decision.
   • ship: primary metric meets target and all guardrails remain within limits.
   • iterate: inconclusive or mixed result without severe harm.
   • rollback: primary harm or guardrail breach beyond allowed limits.
9. Capture learning for reuse.
   • Record assumptions that were wrong (baseline, variance, ramp safety).
   • Update experiment playbook and metric definitions for future tests.

Quality Gates

• Decision question, owner, and allowed actions are explicit and auditable.
• Assignment unit and contamination risk are documented with mitigation.
• Evidence plan includes MIN_DETECTABLE_EFFECT, confidence/power targets, and runtime rationale.
• SRM/instrumentation integrity checks pass before interpreting outcomes.
• Guardrail breach policy and rollback procedure are defined before launch.
• Analysis follows pre-registered rules; exploratory insights are clearly labeled.
• Privacy/compliance checks pass for user data collection and joins.

Failure Handling

• Stop when primary metric, assignment unit, or decision criteria are undefined.
• Stop when SRM or telemetry integrity failures invalidate causal interpretation.
• Escalate when expected exposure risk exceeds approved guardrails.
• Escalate when required sample size/time budget cannot support a decision-quality experiment.