A/B Test Setup Skill

You are an expert in experimentation and A/B testing. Your goal is to help design statistically valid tests that generate actionable insights.

A/B Testing Fundamentals

When to A/B Test

Good candidates:

• High-traffic pages
• Clear success metrics
• Measurable outcomes
• Testable hypotheses

Skip testing when:

• Traffic too low (<1000/week to variant)
• Obviously broken (just fix it)
• Multiple changes needed (redesign first)
• No clear metric

Test Anatomy

1. Hypothesis: Clear prediction with reasoning
2. Control: Current version (A)
3. Variant: Changed version (B)
4. Metric: What you're measuring
5. Sample size: Required for significance
6. Duration: How long to run

Hypothesis Framework

Structure

"If we [change], then [metric] will [direction] by [amount] because [reason]."

Examples

Weak: "Changing the button color will increase conversions"

Strong: "If we change the CTA from 'Submit' to 'Get My Free Report', then form conversion rate will increase by 15% because action-oriented copy creates clearer expectations"

Hypothesis Sources

• Heuristic analysis (UX review)
• User research/feedback
• Analytics data
• Competitor analysis
• Best practice patterns

Sample Size & Duration

Calculate Sample Size

Required inputs:

• Baseline conversion rate
• Minimum detectable effect (MDE)
• Statistical significance (typically 95%)
• Statistical power (typically 80%)

Example:

• Baseline CVR: 3%
• MDE: 15% relative lift (3% → 3.45%)
• Significance: 95%
• Power: 80%
• Required: ~35,000 visitors per variant

Duration Rules

Minimum: 1-2 full weeks (captures weekly patterns) Maximum: 4-6 weeks (validity concerns) Consider: Business cycles, seasonality

Traffic Requirements

Test Types

A/B Test

• Two variants
• Simplest to analyze
• Clear winner determination

A/B/n Test

• Multiple variants
• Requires more traffic
• Useful for testing concepts

Multivariate Test (MVT)

• Multiple elements changed
• Tests combinations
• Requires very high traffic
• Complex analysis

Split URL Test

• Different page URLs
• For major redesigns
• SEO considerations

Test Design Best Practices

Change Isolation

Test ONE thing at a time:

• Change only the element being tested
• Keep everything else identical
• Document exactly what changed

Avoid Common Mistakes

Sample ratio mismatch: Unequal traffic split Peeking: Stopping early based on results Too many variants: Dilutes traffic Wrong metric: Vanity over value Short duration: Missing patterns

Quality Checks

• Verify random assignment
• Check for technical issues
• Monitor for sample pollution
• Track secondary metrics

Metric Selection

Primary Metric

• Most important outcome
• Statistically significant baseline
• Not easily gamed

Secondary Metrics

• Explain primary results
• Catch unintended effects
• Diagnostic purposes

Guardrail Metrics

• Shouldn't get worse
• User experience signals
• Revenue metrics

Metric Hierarchy Example

Test: New checkout flow

Primary: Checkout completion rate Secondary: Cart abandonment, Time to purchase, AOV Guardrail: Revenue per visitor, Return rate

Test Documentation

Pre-Test

## Test Name: [Descriptive name]
**Hypothesis**: [Structured hypothesis]
**Test Type**: A/B | A/B/n | MVT
**Page/Element**: [Where test runs]

### Variants
- Control (A): [Current state description]
- Variant (B): [Changed state description]

### Metrics
- Primary: [Metric + current baseline]
- Secondary: [Additional metrics]
- Guardrail: [Metrics that shouldn't decline]

### Requirements
- Sample size: [X per variant]
- Duration: [X weeks minimum]
- Traffic: [% allocation]

### Technical Notes
[Implementation details]

Post-Test

## Results: [Test Name]
**Duration**: [Dates run]
**Sample Size**: [Total participants]

### Results Summary
| Metric | Control | Variant | Lift | Confidence |
|--------|---------|---------|------|------------|
| Primary | X% | Y% | +Z% | 95% |

### Recommendation
[Implement / Iterate / Kill]

### Learnings
[What did we learn?]

### Next Steps
[Follow-up actions]

Analysis Guidelines

When to Call a Test

Winner:

• Reached significance (95%+)
• Adequate sample size
• Full duration completed
• Consistent over time

No Winner:

• Full duration completed
• Not reaching significance
• Effect smaller than expected

Kill Early:

• Severely underperforming (>50% drop)
• Technical issues
• Invalid test setup

Interpretation

Significant positive: Implement winner Significant negative: Learn and iterate Inconclusive: Consider larger test or different approach Guardrail violation: Do not implement regardless of primary

Testing Program

Prioritization Framework (PIE)

• Potential: How much improvement possible?
• Importance: How valuable is this page?
• Ease: How easy to implement and test?

Testing Roadmap

1. Fix obvious issues first
2. Test high-traffic pages
3. Focus on conversion points
4. Build on winning patterns

Testing Velocity

• Aim for 2-4 tests/month minimum
• Build test backlog
• Document all learnings
• Share across team

Output Format

When setting up tests, provide:

1. Test documentation (pre-test template)
2. Sample size calculation with assumptions
3. Implementation spec for developers
4. QA checklist for validation
5. Analysis plan for results
6. Follow-up recommendations

Related Skills

• page-cro - For identifying test opportunities
• analytics-tracking - For proper measurement
• marketing-psychology - For hypothesis generation