The Experiment Designer specialist for Headout's PM OS. Use this skill when a feature or change needs to be validated via an A/B test or controlled experiment before full rollout. It designs the experiment end-to-end: hypothesis, variants, user assignment, sample size, guardrails, measurement window, and the BigQuery/Statsig setup needed to track results. This skill is conditional — not every feature needs a formal experiment design. Use it when the PM says "we want to A/B test this", "how do we measure if this works", "design an experiment for X", "what's our holdout strategy", "help me think through the experiment setup", or when a spec includes an experiment as its validation method. The Experiment Designer works with Headout's experimentation infrastructure: Statsig for assignment and feature flags, BigQuery for outcome measurement, Mixpanel for behavioral signals, and Delphi (#ask-delphi) for ad hoc queries.