signals-scout-experiments

Detects validity threats and lifecycle drift in running PostHog A/B experiments.

713|118|Updated Aug 11, 2020
One-click install
npx skills add https://github.com/PostHog/posthog-foss --skill signals-scout-experiments
Or copy as Structured Prompt for Agent
Please help me install this Agent Skill.
Skill: signals-scout-experiments
Source: https://github.com/PostHog/posthog-foss/tree/main/products/signals/skills/signals-scout-experiments
Command: npx skills add https://github.com/PostHog/posthog-foss --skill signals-scout-experiments

SYSTEM DOCUMENTATION & REQUIREMENTS

What problem does it solve?

Running A/B experiments silently break: sample ratio mismatch biases results, identity fragmentation contaminates variants, exposure stalls mean the experiment measures nothing, and mid-run flag edits rebucket users. This Skill continuously audits PostHog experiments so teams stop shipping decisions on corrupted data.

Core Features & Use Cases

  • Validity threat detection: Flags sample ratio mismatch (SRM) via native chi-squared p-values, elevated $multiple contamination, exposure stalls, mid-run feature flag mutations, and metrics that structurally cannot answer the hypothesis.
  • Lifecycle hygiene: Identifies zombie experiments running past their useful life, stopped experiments whose flags still serve multiple variants, and stale drafts, bundled into prioritized recommendations.
  • Report authoring with dedupe: Authors or edits inbox reports end-to-end with priorities, suggested reviewers, and chart attachments, using scratchpad memory to avoid duplicate findings across runs.
  • Use Case: A team runs a checkout experiment for weeks; the scout detects a 56/44 split on 22k exposures starting at a flag edit, files a P2 report naming the experiment id, flag key, and onset date, and routes it to the experiment owner.

Quick Start

Ask the agent to audit all running PostHog experiments for sample ratio mismatch, contamination, and exposure stalls, and file reports for any confirmed validity threats.

Frequently Asked Questions about signals-scout-experiments

High-intent search queries and answers about installing and using this skill.

FAQPage Schema
How do I detect sample ratio mismatch in PostHog experiments?

Read the sample_ratio_mismatch.p_value from experiment-results-get, which runs a native chi-squared test excluding $multiple. Flag SRM only when p < 0.01 at healthy volume, and never below roughly 1,000 bucketed exposures or inside the 3-sigma noise band.

What does a high $multiple percentage mean in an A/B test?

A high $multiple share means users saw more than one variant, caused by identity fragmentation, bootstrap versus /decide disagreement, or mid-run flag edits that rebucketed users. Above 0.5% sustained is worth surfacing; above 0.1% matters on uneven splits with exclude handling.

Why does my running experiment show zero exposures?

Zero exposures usually mean broken wiring: the SDK method used does not record $feature_flag_called, the flag is at 0% rollout or inactive, or a custom exposure event is missing its $feature/<flag-key> property. Check the flag's active state first, since paused experiments legitimately have no fresh exposures.

Does this skill judge which experiment variant is winning?

No. Metric movement is explicitly out of scope; variant win or loss is the team's decision. The skill only audits measurement machinery such as SRM, contamination, stalls, and metrics that return no data in any arm.

When should an experiment be flagged as a zombie?

An experiment is a zombie when it runs far past its useful life, such as exposures far above recommended_sample_size or over 60 days with a plateaued exposure curve. These become P3 recommendations to decide or end, bundled into one hygiene finding.

Can mid-run feature flag edits invalidate experiment results?

Yes. Variant rollout redistribution, overall rollout decreases, release-condition tightening, and bucketing-key changes all rebucket users and bias post-edit data. The skill dates these via feature-flags-activity-retrieve; rollout increases are the one safe mid-run change.