ab-test-setup

The saved excerpt is a snapshot from review. The external source remains the complete and most current version.

---
name: "ab-test-setup"
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," "hypothesis," "conversion experiment," "statistical significance," or "test this." For tracking implementation, see analytics-tracking.
license: MIT
metadata:
  version: 1.0.0
  author: Alireza Rezvani
  category: marketing
  updated: 2026-03-06
---

# A/B Test Setup

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

## Initial Assessment

**Check for product marketing context first:**
If `.claude/product-marketing-context.md` exists, read it before asking questions. Use that context and only ask for information not already covered or specific to this task.

Before designing a test, understand:

1. **Test Context** - What are you trying to improve? What change are you considering?
2. **Current State** - Baseline conversion rate? Current traffic volume?
3. **Constraints** - Technical complexity? Timeline? Tools available?

---

## Core Principles

### 1. Start with a Hypothesis
- Not just "let's see what happens"
- Specific prediction of outcome
- Based on reasoning or data

### 2. Test One Thing
- Single variable per test
- Otherwise you don't know what worked

### 3. Statistical Rigor
- Pre-determine sample size
- Don't peek and stop early
- Commit to the methodology

### 4. Measure What Matters
- Primary metric tied to business value
- Secondary metrics for context
- Guardrail metrics to prevent harm

---

## Hypothesis Framework

### Structure

```
Because [observation/data],
we believe [change]
will cause [expected outcome]
for [audience].
We'll know this is true when [metrics].
```

### Example

**Weak**: "Changing the button color might increase clicks."

**Strong**: "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."

---

## Test Types

| Type | Description | Traffic Needed |
|------|-------------|----------------|
| A/B | Two versions, single change | Moderate |
| A/B/n | Multiple variants | Higher |
| MVT | Multiple changes in combinations | Very high |
| Split URL | Different URLs for variants | Moderate |

---

## Sample Size

### Calculate It (bundled tool)

Use this skill's own calculator — don't eyeball it:

```bash
python3 scripts/sample_size_calculator.py --baseline 0.05 --mde 0.20            # human-readable
python3 scripts/sample_size_calculator.py --baseline 0.05 --mde 0.20 --json    # for pipelines
python3 scripts/sample_size_calculator.py --baseline 0.05 --mde 0.20 --daily-traffic 2000  # adds test-duration estimate
```

Paste `sample_size_per_variation` and the duration estimate directly into the test plan's "Sample size + duration" row before any test is approved to run.

### Quick Reference

Generated by `sample_size_calculator.py` (two-proportion z-test, α=0.05 two-tailed, 80% power; relative MDE):

| Baseline | 10% Lift | 20% Lift | 50% Lift |
|----------|----------|----------|----------|
| 1% | 163k/variant | 43k/variant | 7.7k/variant |
| 3% | 53k/variant | 14k/variant | 2.5k/variant |
| 5% | 31k/variant | 8.2k/variant | 1.5k/variant |
| 10% | 15k/variant | 3.8k/variant | 683/variant |

**Cross-check calculators** (should agree with the script within rounding):
- [Evan Miller's](https://www.evanmiller.org/ab-testing/sample-size.html)
- [Optimizely's](https://www.optimizely.com/sample-size-calculator/)

**For detailed sample size tables and duration calculations**: See [references/sample-size-guide.md](references/sample-size-guide.md)

---

## Metrics Selection

### Primary Metric
- Single metric that matters most
- Directly tied to hypothesis
- What you'll use to