Create SKILL.md

New skill: experiment-designer

skills/experiment-designer/SKILL.md

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+---
+name: experiment-designer
+description: Designs A/B tests from hypotheses and interprets experiment results 
+with statistical rigour. Use when user says "run an experiment", "design an A/B 
+test", "test this feature", "interpret these results", "was this experiment 
+successful", or "what sample size do I need".
+metadata:
+  author: Mohit Aggarwal
+  version: 1.0.0
+  category: data-and-metrics
+  tags: [experimentation, data, analytics, ab-testing]
+  documentation: https://github.com/mohitagw15856/pm-claude-skills
+---
+# Experiment Designer Skill
+
+## Purpose
+Produce rigorous experiment designs from product hypotheses, and interpret 
+results with statistical and practical significance — so you can defend every 
+decision to a sceptical engineering lead or data scientist.
+
+## Two-Phase Process
+
+### Phase 1: Experiment Design
+**Required inputs:** hypothesis, primary metric, current baseline, minimum 
+detectable effect (MDE), available sample size per day.
+
+**Output:**
+- Hypothesis restated as: "If we [change], we expect [metric] to [move by X%] 
+  because [reason]"
+- Control and variant definitions
+- Primary metric (one only)
+- Secondary guardrail metrics (2-3 max)
+- Required sample size (calculated from MDE and baseline)
+- Estimated run time in days
+- Pre-defined success criteria (before the test runs — no moving goalposts)
+- Design risk flags: novelty effects, seasonal confounds, multiple testing issues,
+  network effects, sample ratio mismatch risks
+
+### Phase 2: Results Interpretation
+**Required inputs:** control results, variant results, p-value or raw numbers, 
+run duration, any anomalies observed.
+
+**Output:**
+- Statistical significance assessment (p < 0.05 threshold)
+- Practical significance: was the lift meaningful for the business, not just real?
+- Confidence interval interpretation
+- Confounding factors to investigate
+- Recommendation: Ship / Iterate / Kill / Run follow-up test
+- If "Iterate": specific hypotheses to test next
+
+## Quality Checks
+- Never interpret results from an underpowered test without flagging it
+- Always distinguish statistical from practical significance
+- Flag if test was stopped early (peeking problem)
+- Note if sample ratio mismatch occurred