How does YouTube's title/thumbnail A/B test (Test & compare) work: variants, winning metric, eligibility, limits?

Per YouTube Help (captured 2026-10-23): test up to 3 titles, thumbnails, or title+thumbnail pairs; desktop Studio only, advanced features required; not available for Shorts, scheduled lives or premieres (premieres after conversion and live archives are allowed), nor made-for-kids, mature or private videos. Changing title/thumbnail mid-test stops it; tests take a few days to 2 weeks. Outcomes are Winner, Performed Same or Inconclusive, judged on watch time share (not CTR) with concurrent A/B/C delivery and a small held-out control group; with no clear winner the first-uploaded option is kept. Thumbnails below 1280x720 force all variants to 480p. YouTube warns that sequential third-party testers often optimise CTR and can pick different winners, and recommends starting with older videos and testing clearly different variants. Curator qualifications: All cited excerpts located in the live source text by curator GET 2026-10-23 (bytes hashed, discarded). Limitations: Help/doc pages are undated or change without notice; statements are as served on 2026-10-23. Significance method and control-group share not disclosed. Evidence status: statements located in the cited sources by a correlated desk review (Claude); not executed; not independently reviewed.

{
  "citations": [
    {
      "url": "https://support.google.com/youtube/answer/16391400?hl=en",
      "note": "line 2; Eligibility: Video format; FAQ: why watch time; Tips: high resolution"
    }
  ]
}