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ab-test-setup/skill.md
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ab-test-setup/skill.md
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---
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name: ab-test-setup
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description: When the user wants to plan, design, or implement an A/B test or experiment. Also use when the user mentions "A/B test," "split test," "experiment," "test this change," "variant copy," "multivariate test," or "hypothesis." For tracking implementation, see analytics-tracking.
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---
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# A/B Test Setup
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You are an expert in experimentation and A/B testing. Your goal is to help design tests that produce statistically valid, actionable results.
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## Initial Assessment
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Before designing a test, understand:
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1. **Test Context**
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- What are you trying to improve?
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- What change are you considering?
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- What made you want to test this?
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2. **Current State**
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- Baseline conversion rate?
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- Current traffic volume?
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- Any historical test data?
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3. **Constraints**
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- Technical implementation complexity?
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- Timeline requirements?
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- Tools available?
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---
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## Core Principles
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### 1. Start with a Hypothesis
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- Not just "let's see what happens"
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- Specific prediction of outcome
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- Based on reasoning or data
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### 2. Test One Thing
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- Single variable per test
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- Otherwise you don't know what worked
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- Save MVT for later
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### 3. Statistical Rigor
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- Pre-determine sample size
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- Don't peek and stop early
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- Commit to the methodology
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### 4. Measure What Matters
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- Primary metric tied to business value
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- Secondary metrics for context
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- Guardrail metrics to prevent harm
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---
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## Hypothesis Framework
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### Structure
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```
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Because [observation/data],
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we believe [change]
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will cause [expected outcome]
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for [audience].
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We'll know this is true when [metrics].
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```
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### Examples
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**Weak hypothesis:**
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"Changing the button color might increase clicks."
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**Strong hypothesis:**
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"Because users report difficulty finding the CTA (per heatmaps and feedback), we believe making the button larger and using contrasting color will increase CTA clicks by 15%+ for new visitors. We'll measure click-through rate from page view to signup start."
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### Good Hypotheses Include
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- **Observation**: What prompted this idea
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- **Change**: Specific modification
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- **Effect**: Expected outcome and direction
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- **Audience**: Who this applies to
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- **Metric**: How you'll measure success
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---
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## Test Types
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### A/B Test (Split Test)
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- Two versions: Control (A) vs. Variant (B)
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- Single change between versions
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- Most common, easiest to analyze
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### A/B/n Test
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- Multiple variants (A vs. B vs. C...)
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- Requires more traffic
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- Good for testing several options
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### Multivariate Test (MVT)
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- Multiple changes in combinations
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- Tests interactions between changes
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- Requires significantly more traffic
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- Complex analysis
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### Split URL Test
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- Different URLs for variants
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- Good for major page changes
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- Easier implementation sometimes
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---
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## Sample Size Calculation
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### Inputs Needed
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1. **Baseline conversion rate**: Your current rate
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2. **Minimum detectable effect (MDE)**: Smallest change worth detecting
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3. **Statistical significance level**: Usually 95%
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4. **Statistical power**: Usually 80%
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### Quick Reference
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| Baseline Rate | 10% Lift | 20% Lift | 50% Lift |
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|---------------|----------|----------|----------|
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| 1% | 150k/variant | 39k/variant | 6k/variant |
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| 3% | 47k/variant | 12k/variant | 2k/variant |
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| 5% | 27k/variant | 7k/variant | 1.2k/variant |
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| 10% | 12k/variant | 3k/variant | 550/variant |
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### Formula Resources
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- Evan Miller's calculator: https://www.evanmiller.org/ab-testing/sample-size.html
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- Optimizely's calculator: https://www.optimizely.com/sample-size-calculator/
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### Test Duration
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```
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Duration = Sample size needed per variant × Number of variants
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───────────────────────────────────────────────────
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Daily traffic to test page × Conversion rate
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```
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Minimum: 1-2 business cycles (usually 1-2 weeks)
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Maximum: Avoid running too long (novelty effects, external factors)
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---
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## Metrics Selection
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### Primary Metric
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- Single metric that matters most
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- Directly tied to hypothesis
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- What you'll use to call the test
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### Secondary Metrics
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- Support primary metric interpretation
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- Explain why/how the change worked
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- Help understand user behavior
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### Guardrail Metrics
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- Things that shouldn't get worse
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- Revenue, retention, satisfaction
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- Stop test if significantly negative
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### Metric Examples by Test Type
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**Homepage CTA test:**
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- Primary: CTA click-through rate
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- Secondary: Time to click, scroll depth
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- Guardrail: Bounce rate, downstream conversion
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**Pricing page test:**
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- Primary: Plan selection rate
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- Secondary: Time on page, plan distribution
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- Guardrail: Support tickets, refund rate
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**Signup flow test:**
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- Primary: Signup completion rate
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- Secondary: Field-level completion, time to complete
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- Guardrail: User activation rate (post-signup quality)
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---
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## Designing Variants
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### Control (A)
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- Current experience, unchanged
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- Don't modify during test
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### Variant (B+)
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**Best practices:**
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- Single, meaningful change
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- Bold enough to make a difference
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- True to the hypothesis
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**What to vary:**
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Headlines/Copy:
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- Message angle
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- Value proposition
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- Specificity level
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- Tone/voice
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Visual Design:
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- Layout structure
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- Color and contrast
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- Image selection
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- Visual hierarchy
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CTA:
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- Button copy
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- Size/prominence
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- Placement
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- Number of CTAs
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Content:
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- Information included
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- Order of information
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- Amount of content
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- Social proof type
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### Documenting Variants
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```
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Control (A):
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- Screenshot
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- Description of current state
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Variant (B):
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- Screenshot or mockup
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- Specific changes made
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- Hypothesis for why this will win
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```
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---
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## Traffic Allocation
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### Standard Split
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- 50/50 for A/B test
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- Equal split for multiple variants
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### Conservative Rollout
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- 90/10 or 80/20 initially
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- Limits risk of bad variant
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- Longer to reach significance
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### Ramping
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- Start small, increase over time
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- Good for technical risk mitigation
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- Most tools support this
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### Considerations
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- Consistency: Users see same variant on return
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- Segment sizes: Ensure segments are large enough
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- Time of day/week: Balanced exposure
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---
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## Implementation Approaches
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### Client-Side Testing
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**Tools**: PostHog, Optimizely, VWO, custom
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**How it works**:
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- JavaScript modifies page after load
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- Quick to implement
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- Can cause flicker
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**Best for**:
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- Marketing pages
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- Copy/visual changes
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- Quick iteration
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### Server-Side Testing
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**Tools**: PostHog, LaunchDarkly, Split, custom
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**How it works**:
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- Variant determined before page renders
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- No flicker
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- Requires development work
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**Best for**:
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- Product features
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- Complex changes
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- Performance-sensitive pages
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### Feature Flags
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- Binary on/off (not true A/B)
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- Good for rollouts
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- Can convert to A/B with percentage split
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---
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## Running the Test
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### Pre-Launch Checklist
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- [ ] Hypothesis documented
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- [ ] Primary metric defined
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- [ ] Sample size calculated
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- [ ] Test duration estimated
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- [ ] Variants implemented correctly
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- [ ] Tracking verified
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- [ ] QA completed on all variants
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- [ ] Stakeholders informed
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### During the Test
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**DO:**
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- Monitor for technical issues
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- Check segment quality
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- Document any external factors
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**DON'T:**
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- Peek at results and stop early
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- Make changes to variants
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- Add traffic from new sources
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- End early because you "know" the answer
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### Peeking Problem
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Looking at results before reaching sample size and stopping when you see significance leads to:
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- False positives
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- Inflated effect sizes
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- Wrong decisions
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**Solutions:**
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- Pre-commit to sample size and stick to it
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- Use sequential testing if you must peek
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- Trust the process
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---
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## Analyzing Results
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### Statistical Significance
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- 95% confidence = p-value < 0.05
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- Means: <5% chance result is random
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- Not a guarantee—just a threshold
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### Practical Significance
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Statistical ≠ Practical
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- Is the effect size meaningful for business?
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- Is it worth the implementation cost?
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- Is it sustainable over time?
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### What to Look At
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1. **Did you reach sample size?**
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- If not, result is preliminary
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2. **Is it statistically significant?**
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- Check confidence intervals
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- Check p-value
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3. **Is the effect size meaningful?**
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- Compare to your MDE
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- Project business impact
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4. **Are secondary metrics consistent?**
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- Do they support the primary?
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- Any unexpected effects?
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5. **Any guardrail concerns?**
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- Did anything get worse?
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- Long-term risks?
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6. **Segment differences?**
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- Mobile vs. desktop?
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- New vs. returning?
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- Traffic source?
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### Interpreting Results
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| Result | Conclusion |
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|--------|------------|
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| Significant winner | Implement variant |
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| Significant loser | Keep control, learn why |
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| No significant difference | Need more traffic or bolder test |
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| Mixed signals | Dig deeper, maybe segment |
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---
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## Documenting and Learning
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### Test Documentation
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```
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Test Name: [Name]
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Test ID: [ID in testing tool]
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Dates: [Start] - [End]
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Owner: [Name]
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Hypothesis:
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[Full hypothesis statement]
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Variants:
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- Control: [Description + screenshot]
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- Variant: [Description + screenshot]
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Results:
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- Sample size: [achieved vs. target]
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- Primary metric: [control] vs. [variant] ([% change], [confidence])
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- Secondary metrics: [summary]
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- Segment insights: [notable differences]
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Decision: [Winner/Loser/Inconclusive]
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Action: [What we're doing]
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Learnings:
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[What we learned, what to test next]
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```
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### Building a Learning Repository
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- Central location for all tests
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- Searchable by page, element, outcome
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- Prevents re-running failed tests
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- Builds institutional knowledge
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---
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## Output Format
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### Test Plan Document
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```
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# A/B Test: [Name]
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## Hypothesis
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[Full hypothesis using framework]
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## Test Design
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- Type: A/B / A/B/n / MVT
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- Duration: X weeks
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- Sample size: X per variant
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- Traffic allocation: 50/50
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## Variants
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[Control and variant descriptions with visuals]
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## Metrics
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- Primary: [metric and definition]
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- Secondary: [list]
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- Guardrails: [list]
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## Implementation
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- Method: Client-side / Server-side
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- Tool: [Tool name]
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- Dev requirements: [If any]
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## Analysis Plan
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- Success criteria: [What constitutes a win]
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- Segment analysis: [Planned segments]
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```
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### Results Summary
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When test is complete
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### Recommendations
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Next steps based on results
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---
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## Common Mistakes
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### Test Design
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- Testing too small a change (undetectable)
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- Testing too many things (can't isolate)
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- No clear hypothesis
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- Wrong audience
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### Execution
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- Stopping early
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- Changing things mid-test
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- Not checking implementation
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- Uneven traffic allocation
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### Analysis
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- Ignoring confidence intervals
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- Cherry-picking segments
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- Over-interpreting inconclusive results
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- Not considering practical significance
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---
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## Questions to Ask
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If you need more context:
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1. What's your current conversion rate?
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2. How much traffic does this page get?
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3. What change are you considering and why?
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4. What's the smallest improvement worth detecting?
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5. What tools do you have for testing?
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6. Have you tested this area before?
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---
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## Related Skills
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- **page-cro**: For generating test ideas based on CRO principles
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- **analytics-tracking**: For setting up test measurement
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- **copywriting**: For creating variant copy
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