A/B Test Setup

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name: ab-test-setup description: Structured guide for setting up A/B tests with mandatory gates for hypothesis, metrics, and execution readiness. Use when planning experiments, defining test hypotheses, calculating sample sizes, or validating A/B test designs. tags: [ab-testing, experimentation, cro]

A/B Test Setup

Purpose & Scope

Ensure every A/B test is valid, rigorous, and safe before a single line of code is written.

• Prevents "peeking"
• Enforces statistical power
• Blocks invalid hypotheses

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Pre-Requisites

You must have:

• A clear user problem
• Access to an analytics source
• Roughly estimated traffic volume

Hypothesis Quality Checklist

A valid hypothesis includes:

• Observation or evidence
• Single, specific change
• Directional expectation
• Defined audience
• Measurable success criteria

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Hypothesis Lock (Hard Gate)

Before designing variants or metrics, you MUST:

• Present the final hypothesis
• Specify:
  • Target audience
  • Primary metric
  • Expected direction of effect
  • Minimum Detectable Effect (MDE)

Ask explicitly:

"Is this the final hypothesis we are committing to for this test?"

Do NOT proceed until confirmed.

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Assumptions & Validity Check (Mandatory)

Explicitly list assumptions about:

• Traffic stability
• User independence
• Metric reliability
• Randomization quality
• External factors (seasonality, campaigns, releases)

If assumptions are weak or violated:

• Warn the user
• Recommend delaying or redesigning the test

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Test Type Selection

Choose the simplest valid test:

• A/B Test - single change, two variants
• A/B/n Test - multiple variants, higher traffic required
• Multivariate Test (MVT) - interaction effects, very high traffic
• Split URL Test - major structural changes

Default to A/B unless there is a clear reason otherwise.

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Metrics Definition

Primary Metric (Mandatory)

• Single metric used to evaluate success
• Directly tied to the hypothesis
• Pre-defined and frozen before launch

Secondary Metrics

• Provide context
• Explain why results occurred
• Must not override the primary metric

Guardrail Metrics

• Metrics that must not degrade
• Used to prevent harmful wins
• Trigger test stop if significantly negative

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Sample Size & Duration

Define upfront:

• Baseline rate
• MDE
• Significance level (typically 95%)
• Statistical power (typically 80%)

Estimate:

• Required sample size per variant
• Expected test duration

Do NOT proceed without a realistic sample size estimate.

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Execution Readiness Gate (Hard Stop)

You may proceed to implementation only if all are true:

• Hypothesis is locked
• Primary metric is frozen
• Sample size is calculated
• Test duration is defined
• Guardrails are set
• Tracking is verified

If any item is missing, stop and resolve it.

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Running the Test

During the Test

DO:

• Monitor technical health
• Document external factors

DO NOT:

• Stop early due to "good-looking" results
• Change variants mid-test
• Add new traffic sources
• Redefine success criteria

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Analyzing Results

Analysis Discipline

When interpreting results:

• Do NOT generalize beyond the tested population
• Do NOT claim causality beyond the tested change
• Do NOT override guardrail failures
• Separate statistical significance from business judgment

Interpretation Outcomes

Result	Action
Significant positive	Consider rollout
Significant negative	Reject variant, document learning
Inconclusive	Consider more traffic or bolder change
Guardrail failure	Do not ship, even if primary wins

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Documentation & Learning

Test Record (Mandatory)

Document:

• Hypothesis
• Variants
• Metrics
• Sample size vs achieved
• Results
• Decision
• Learnings
• Follow-up ideas

Store records in a shared, searchable location to avoid repeated failures.

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Refusal Conditions (Safety)

Refuse to proceed if:

• Baseline rate is unknown and cannot be estimated
• Traffic is insufficient to detect the MDE
• Primary metric is undefined
• Multiple variables are changed without proper design
• Hypothesis cannot be clearly stated

Explain why and recommend next steps.

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Key Principles (Non-Negotiable)

• One hypothesis per test
• One primary metric
• Commit before launch
• No peeking
• Learning over winning
• Statistical rigor first

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Final Reminder

A/B testing is not about proving ideas right. It is about learning the truth with confidence.

If you feel tempted to rush, simplify, or "just try it" -- that is the signal to slow down and re-check the design.