What problem does it solve? Cold email operators often change the list, copy, and offer at the same time, which destroys all learning signal. This Skill enforces single-variable experiment design so every campaign test produces an attributable, trustworthy result. ## Core Features & Use Cases - Three Experiment Types: Defines list-only, copy-only, and combined experiments with explicit confidence weighting for each. - Sample Size & Success Criteria: Provides minimum sends-per-arm tables based on baseline reply rate and expected lift, plus pre-registered success/failure thresholds. - Structured Output: Produces a YAML experiment plan file with hypothesis, constants, arms, and results fields for tracking through day-21 measurement. - Use Case: You want to test whether Heads of Marketing respond better than VP Sales. The Skill walks you through writing a one-sentence hypothesis, locking all other variables constant, calculating that you need roughly 2,000 sends per arm, and defining the win threshold before launch. ## Quick Start Ask the AI to design a single-variable experiment testing a new target list against your current baseline campaign.